Track client side video quality and performance

[mithro]

We should be collecting video quality and performance stats on the client side and then report on them. Maybe through Google Analytics?

Firefox

• mozParsedFrames - number of frames that have been demuxed and
  extracted out of the media.
• mozDecodedFrames - number of frames that have been decoded -
  converted into YCbCr.
• mozPresentedFrames - number of frames that have been presented to
  the rendering pipeline for rendering - were "set as the current
  image".
• mozPaintedFrames - number of frames which were presented to the
  rendering pipeline and ended up being painted on the screen. Note that
  if the video is not on screen (e.g. in another tab or scrolled off
  screen), this counter will not increase.
• mozFrameDelay - the time delay between presenting the last frame
  and it being painted on screen (approximately).

Webkit

• webkitAudioBytesDecoded - number of audio bytes that have been decoded.
• webkitVideoBytesDecoded - number of video bytes that have been decoded.
• webkitDecodedFrames - number of frames that have been demuxed and
  extracted out of the media.
• webkitDroppedFrames - number of frames that were decoded but not
  displayed due to performance issues.