Add blink_gc_plugin variable to enable the clang plugin for checking GC consistency.

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

Review URL: https://codereview.chromium.org/133473002

git-svn-id: svn://svn.chromium.org/blink/trunk@166005 bbb929c8-8fbe-4397-9dbb-9b2b20218538