MediaSource must use ref-counting to handle pending activity counting.

Switch MediaSource to deriving from RefCountedGarbageCollected, as
ActiveDOMObject::setPendingActivity() relies on ref-counting.

R=haraken@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/360323002

git-svn-id: svn://svn.chromium.org/blink/trunk@177324 bbb929c8-8fbe-4397-9dbb-9b2b20218538