Fix Oilpan build broken by r178326.

TBR=samli@chromium.org,oilpan-reviews@chromium.org
BUG=

Review URL: https://codereview.chromium.org/403463002

git-svn-id: svn://svn.chromium.org/blink/trunk@178329 bbb929c8-8fbe-4397-9dbb-9b2b20218538