Oilpan: fix build after r183834.

TBR=oilpan-reviews,haraken
BUG=423536
NOTRY=true

Review URL: https://codereview.chromium.org/640813003

git-svn-id: svn://svn.chromium.org/blink/trunk@183837 bbb929c8-8fbe-4397-9dbb-9b2b20218538