Would be useful to know how often users use screen mirroring to watch/listen to web media on external screens (Cast devices, etc).

According to Android's android.app.Presentation (http://developer.android.com/reference/android/app/Presentation.html) it seems like all we need to do is to check the following:

MediaRouter mediaRouter = (MediaRouter) context.getSystemService(Context.MEDIA_ROUTER_SERVICE);
 MediaRouter.RouteInfo route = mediaRouter.getSelectedRoute();
 if (route != null) {
     Display presentationDisplay = route.getPresentationDisplay();
     if (presentationDisplay != null) {
       ... screen mirroring is on
     }
 }

We'll have to filter out Cast devices vs HDMI over USB.

With Spitzer I think we could just record the state when any tab becomes audible. Adding Dale for his thoughts on where the best to plug this check and UMA in (if we could distinguish between audio and video, fs or not, it'd be great).

Perhaps, especially if we already have playback user actions (like "playback started"/"playback stopped") we could add "screen mirroring started" and "screen mirroring stopped" and then get the usage we need from that?

Based on that metric we'd prioritize optimizing media remoting when screen is being mirrored.