Add trace events for MediaSource & GenericEventQueue so it is easier to visualize media perf.

- Adding trace events to track the duration of MediaSource append and remove operations.
- Adding trace events to track when events are enqueued, how long they are pending, and
  how long they take to dispatch.

Review URL: https://codereview.chromium.org/25762004

git-svn-id: svn://svn.chromium.org/blink/trunk@158887 bbb929c8-8fbe-4397-9dbb-9b2b20218538