Oilpan: Build fix after r170389

r170389 broke oilpan's build (Event related.) This CL rights the ship.

TBR=haraken@chromium.org,ager@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/216233013

git-svn-id: svn://svn.chromium.org/blink/trunk@170411 bbb929c8-8fbe-4397-9dbb-9b2b20218538