Oilpan: fix build after r183913.

TBR=oilpan-reviews,haraken
BUG=391288
NOTRY=true

Review URL: https://codereview.chromium.org/659953004

git-svn-id: svn://svn.chromium.org/blink/trunk@183928 bbb929c8-8fbe-4397-9dbb-9b2b20218538