Use GenericEventQueue class in MediaController

Use GenericEventQueue class in MediaController to make the event dispatching
code simpler.

R=esprehn@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/258563005

git-svn-id: svn://svn.chromium.org/blink/trunk@172566 bbb929c8-8fbe-4397-9dbb-9b2b20218538