Based on the conversation in #4243 this implements a way to increase
and decrease the size of the thread pool used in v8.

Currently v8 restricts the thread pool size to `kMaxThreadPoolSize`
which at this commit is (4). So it is only possible to
decrease the thread pool size at the time of this commit. However with
changes upstream this could change at a later date.
If set to 0 then v8 would choose an appropriate size of the thread pool
based on the number of online processors.

PR-URL: #4344
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Based on the conversation in #4243 this implements a way to increase
and decrease the size of the thread pool used in v8.

Currently v8 restricts the thread pool size to `kMaxThreadPoolSize`
which at this commit is (4). So it is only possible to
decrease the thread pool size at the time of this commit. However with
changes upstream this could change at a later date.
If set to 0 then v8 would choose an appropriate size of the thread pool
based on the number of online processors.

PR-URL: #4344
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Notable changes:

* buffer:
  * make byteLength work with ArrayBuffer & DataView (Jackson Tian)
[#5255](#5255)
  * backport --zero-fill-buffers command line option (James M Snell)
[#5744](#5744)
  * backport new buffer constructor APIs (James M Snell)
[#5763](#5763)
  * add swap16() and swap32() methods (James M Snell)
[#5724](#5724)
* fs: add the fs.mkdtemp() function. (Florian MARGAINE)
[#5333](#5333)
* net: emit host in lookup event (HUANG Wei)
[#5598](#5598)
* node: --no-browser-globals configure flag (Fedor Indutny)
[#5853](#5853)
* npm: Upgrade to v3.8.3. (Forrest L Norvell)
* repl: support standalone blocks (Prince J Wesley)
[#5581](#5581)
* src: override v8 thread defaults using cli options (Tom Gallacher)
[#4344](#4344)

Notable changes:

* buffer:
  * make byteLength work with ArrayBuffer & DataView (Jackson Tian)
[#5255](#5255)
  * backport --zero-fill-buffers command line option (James M Snell)
[#5744](#5744)
  * backport new buffer constructor APIs (James M Snell)
[#5763](#5763)
  * add swap16() and swap32() methods (James M Snell)
[#5724](#5724)
* fs: add the fs.mkdtemp() function. (Florian MARGAINE)
[#5333](#5333)
* net: emit host in lookup event (HUANG Wei)
[#5598](#5598)
* node: --no-browser-globals configure flag (Fedor Indutny)
[#5853](#5853)
* npm: Upgrade to v3.8.3. Fixes a security flaw in the use of
authentication tokens in HTTP requests that would allow an attacker to
set up a server that could collect tokens from users of the command-line
interface. Authentication tokens have previously been sent with every
request made by the CLI for logged-in users, regardless of the destination
of the request. This update fixes this by only including those tokens
for requests made against the registry or registries used for the
current install. (Forrest L Norvell)
[npm#6](npm#6)
* repl: support standalone blocks (Prince J Wesley)
[#5581](#5581)
* src: override v8 thread defaults using cli options (Tom Gallacher)
[#4344](#4344)

Notable changes:

* buffer:
  * make byteLength work with ArrayBuffer & DataView (Jackson Tian)
[#5255](#5255)
  * backport --zero-fill-buffers command line option (James M Snell)
[#5744](#5744)
  * backport new buffer constructor APIs (James M Snell)
[#5763](#5763)
  * add swap16() and swap32() methods (James M Snell)
[#5724](#5724)
* fs: add the fs.mkdtemp() function. (Florian MARGAINE)
[#5333](#5333)
* net: emit host in lookup event (HUANG Wei)
[#5598](#5598)
* node: --no-browser-globals configure flag (Fedor Indutny)
[#5853](#5853)
* npm: Upgrade to v3.8.3. Fixes a security flaw in the use of
authentication tokens in HTTP requests that would allow an attacker to
set up a server that could collect tokens from users of the command-line
interface. Authentication tokens have previously been sent with every
request made by the CLI for logged-in users, regardless of the destination
of the request. This update fixes this by only including those tokens
for requests made against the registry or registries used for the
current install. (Forrest L Norvell)
[npm#6](npm#6)
* repl: support standalone blocks (Prince J Wesley)
[#5581](#5581)
* src: override v8 thread defaults using cli options (Tom Gallacher)
[#4344](#4344)

Notable changes:

* buffer:
  * make byteLength work with ArrayBuffer & DataView (Jackson Tian)
[#5255](#5255)
  * backport --zero-fill-buffers command line option (James M Snell)
[#5744](#5744)
  * backport new buffer constructor APIs (James M Snell)
[#5763](#5763)
  * add swap16() and swap32() methods (James M Snell)
[#5724](#5724)
* fs: add the fs.mkdtemp() function. (Florian MARGAINE)
[#5333](#5333)
* net: emit host in lookup event (HUANG Wei)
[#5598](#5598)
* node: --no-browser-globals configure flag (Fedor Indutny)
[#5853](#5853)
* npm: Upgrade to v3.8.3. Fixes a security flaw in the use of
authentication tokens in HTTP requests that would allow an attacker to
set up a server that could collect tokens from users of the command-line
interface. Authentication tokens have previously been sent with every
request made by the CLI for logged-in users, regardless of the destination
of the request. This update fixes this by only including those tokens
for requests made against the registry or registries used for the
current install. (Forrest L Norvell)
[npm#6](npm#6)
* repl: support standalone blocks (Prince J Wesley)
[#5581](#5581)
* src: override v8 thread defaults using cli options (Tom Gallacher)
[#4344](#4344)

PR-URL: #5970

fix: nodejs#42523

Problem:
If no platform worker exists, Node.js doesn't shut down when background
tasks exist. It keeps waiting in `NodePlatform::DrainTasks`.

Observation:
It seems that Node.js used to use V8's `DefaultPlatform` implementation,
which chooses a suitable default value in case that `--v8-pool-size=0`
is given as a command-line option. However, Node.js currently uses its
own v8::Platform implementation, `NodePlatform`. It doesn't have the
logic to handle the case.

I referred to nodejs#4344 to track the issue.

fix: nodejs#42523

Problem:
If no platform worker exists, Node.js doesn't shut down when background
tasks exist. It keeps waiting in `NodePlatform::DrainTasks`.

Observation:
It seems that Node.js used to use V8's `DefaultPlatform` implementation,
which chooses a suitable default value in case that `--v8-pool-size=0`
is given as a command-line option. However, Node.js currently uses its
own v8::Platform implementation, `NodePlatform`. It doesn't have the
logic to handle the case.

I referred to nodejs#4344 to track the issue.

fix: nodejs#42523

Problem:
If no platform worker exists, Node.js doesn't shut down when background
tasks exist. It keeps waiting in `NodePlatform::DrainTasks`.

Observation:
It seems that Node.js used to use V8's `DefaultPlatform` implementation,
which chooses a suitable default value in case that `--v8-pool-size=0`
is given as a command-line option. However, Node.js currently uses its
own v8::Platform implementation, `NodePlatform`. It doesn't have the
logic to handle the case.

I referred to nodejs#4344 to track the issue.