Oilpan: Remove resolved GC_PLUGIN_IGNORE.

BUG=367712
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/337603007

git-svn-id: svn://svn.chromium.org/blink/trunk@177276 bbb929c8-8fbe-4397-9dbb-9b2b20218538