Oilpan: Build fix after r168876

r168876 broke compiles of all oilpan bots.
This CL fixes it.

Review URL: https://codereview.chromium.org/194063003

git-svn-id: svn://svn.chromium.org/blink/trunk@168878 bbb929c8-8fbe-4397-9dbb-9b2b20218538