Oilpan: trace MediaControlElement's element reference.

R=ager@chromium.org,haraken@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/385593002

git-svn-id: svn://svn.chromium.org/blink/trunk@177858 bbb929c8-8fbe-4397-9dbb-9b2b20218538