Oilpan: fix build after r181867.

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

Review URL: https://codereview.chromium.org/570453003

git-svn-id: svn://svn.chromium.org/blink/trunk@181870 bbb929c8-8fbe-4397-9dbb-9b2b20218538