Oilpan: fix build after r183074.

TBR=haraken,oilpan-reviews
BUG=419270
NOTRY=true

Review URL: https://codereview.chromium.org/639693002

git-svn-id: svn://svn.chromium.org/blink/trunk@183382 bbb929c8-8fbe-4397-9dbb-9b2b20218538