Oilpan: fix build after r183788.

TBR=oilpan-reviews,haraken
BUG=355467
NOTRY=true

Review URL: https://codereview.chromium.org/660723003

git-svn-id: svn://svn.chromium.org/blink/trunk@183797 bbb929c8-8fbe-4397-9dbb-9b2b20218538