Oilpan: have the media element safely close its MediaSource.

The media element must notify its attached MediaSource upon being
finalized, as the MediaSource must then transition to a closed state
and fire the corresponding event. Hence, upon media element finalization,
it will conditionally notify its MediaSource (if any) object if the
latter survived the GC. If neither survived, an explicit close isn't
needed.

With that in place, the media element no longer keeps a RefPtr to its
MediaSource (with Oilpan enabled.) Consequently, ref-forwarding over
HTMLMediaSource is no longer needed nor supported.

R=haraken@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/363953002

git-svn-id: svn://svn.chromium.org/blink/trunk@177532 bbb929c8-8fbe-4397-9dbb-9b2b20218538