Oilpan: fix build after r178753.

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

Review URL: https://codereview.chromium.org/410933002

git-svn-id: svn://svn.chromium.org/blink/trunk@178757 bbb929c8-8fbe-4397-9dbb-9b2b20218538