Oilpan: fix build after r180648.

TBR=oilpan-reviews,mkwst@chromium.org
BUG=400674
NOTRY=true

Review URL: https://codereview.chromium.org/489903002

git-svn-id: svn://svn.chromium.org/blink/trunk@180674 bbb929c8-8fbe-4397-9dbb-9b2b20218538