Oilpan: fix to satisfy the Blink GC plugin after recent plugin update.

R=wibling@chromium.org
BUG=

Review URL: https://codereview.chromium.org/261963002

git-svn-id: svn://svn.chromium.org/blink/trunk@173206 bbb929c8-8fbe-4397-9dbb-9b2b20218538