Trap hidden in debug gyp settings

toolchain.gypi contains this setting:

          ['OS=="linux" and disable_glibcxx_debug==0', {
            # Enable libstdc++ debugging facilities to help catch problems
            # early, see http://crbug.com/65151 .
                        'defines': ['_GLIBCXX_DEBUG=1',],
          }],

which will increase the size of std::vector, and make this assert in V8 spaces.cc fail:

void Space::CheckOffsetsAreConsistent() const {
  DCHECK_EQ(Space::kIdOffset, OFFSET_OF(Space, id_));
}

# Debug check failed: Space::kIdOffset == (reinterpret_cast<intptr_t>(&(reinterpret_cast<Space*>(16)->id_)) - 16) (72 vs. 104).