Oilpan: Use transition types for HTMLMediaElement and make AudioSourceProviderClient a GC mixin.

BUG=357163
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/294053006

git-svn-id: svn://svn.chromium.org/blink/trunk@176013 bbb929c8-8fbe-4397-9dbb-9b2b20218538