Default enable the Blink GC plugin.

The Blink GC plugin helps to ensure correctness of the GC infrastructure.
Errors are described at: http://www.chromium.org/developers/blink-gc-plugin-errors
The plugin can be disabled using GYP_DEFINE blink_gc_plugin.

R=abarth@chromium.org, ager@chromium.org, thakis@chromium.org
BUG=

Review URL: https://codereview.chromium.org/262533008

git-svn-id: svn://svn.chromium.org/blink/trunk@173367 bbb929c8-8fbe-4397-9dbb-9b2b20218538