Oilpan: fix build after r180544.

TBR=oilpan-reviews
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/488543002

git-svn-id: svn://svn.chromium.org/blink/trunk@180545 bbb929c8-8fbe-4397-9dbb-9b2b20218538