Oilpan: fix build after r179422.

TBR=haraken@chromium.org,oilpan-reviews@chromium.org
BUG=392474
NOTRY=true

Review URL: https://codereview.chromium.org/434203002

git-svn-id: svn://svn.chromium.org/blink/trunk@179441 bbb929c8-8fbe-4397-9dbb-9b2b20218538