Oilpan: fix build after r177984.

TBR=tkent@chromium.org,oilpan-reviews@chromium.org
BUG=388961
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/385343002

git-svn-id: svn://svn.chromium.org/blink/trunk@177994 bbb929c8-8fbe-4397-9dbb-9b2b20218538