Oilpan: fix build after r179903.

TBR=haraken,oilpan-reviews
BUG=396400
NOTRY=true

Review URL: https://codereview.chromium.org/458053004

git-svn-id: svn://svn.chromium.org/blink/trunk@179907 bbb929c8-8fbe-4397-9dbb-9b2b20218538