Oilpan:fix build after r183981.

TBR=oilpan-reviews,haraken
BUG=423823
NOTRY=true

Review URL: https://codereview.chromium.org/665093002

git-svn-id: svn://svn.chromium.org/blink/trunk@183988 bbb929c8-8fbe-4397-9dbb-9b2b20218538