Oilpan: Left-most derive the GC base in HTMLMediaElement.

This is now required by the Blink GC plugin.

R=ager@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/228543002

git-svn-id: svn://svn.chromium.org/blink/trunk@171055 bbb929c8-8fbe-4397-9dbb-9b2b20218538