osr: Add support for video playback status notification

[csmoe]

While using OSR with external-begin-frame, the webcontent frames will only be requested by host app's SendExternalBeginFrame call. So for app not driven its rendering stage with continue timer like vsync, the video playback will be janky. It will be good for those kind app to know when the video is playing/paused, then it can launch a temporary timer to request new frames.

[magreenblatt]

You could probably implement this by injecting JavaScript that hooks all video elements in the page.