Oilpan: fix MediaController crashes after r176358.

MediaController::trace() needs to have the declaration of ExecutionContext
available, so as to determine what NeedsAdjustAndMark<> resolves to for
its (weak) ExecutionContext member.

R=haraken@chromium.org,zerny@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/344443008

git-svn-id: svn://svn.chromium.org/blink/trunk@176415 bbb929c8-8fbe-4397-9dbb-9b2b20218538