Oilpan: fix build after r178304

TBR=haraken@chromium.org,oilpan-reviews@chromium.org
BUG=393595
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/398053006

git-svn-id: svn://svn.chromium.org/blink/trunk@178324 bbb929c8-8fbe-4397-9dbb-9b2b20218538