Should media resource URLs be optional?

[markafoltz]

If a controller has a media element using Media Source Extensions, there may be no media resource URL at all; the application will attach a MediaSource directly to the media element by setting its srcObject.

We need to allow this scenario in the definitions of the remote-playback-* messages, and also clarify how to handle transitions between src (list of media resources) and srcObject in the controlling media element.