Oilpan: remove GlobalFetch GC plugin ignorance.

The latest clang roll (http://crbug.com/444089) includes
the GC plugin fix from 444740. Hence, we can retire the
GC_PLUGIN_IGNORE() and depend on it.

R=haraken
BUG=444740

Review URL: https://codereview.chromium.org/989483002

git-svn-id: svn://svn.chromium.org/blink/trunk@191419 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed