[webrtc-pc] "track" event fires while MediaStream has a partial list of tracks.

taylor-b has just created a new issue for https://github.com/w3c/webrtc-pc:

== "track" event fires while MediaStream has a partial list of tracks. ==
If the spec is interpreted literally, when a remote description with both audio/video tracks is set, you'd first get a `track` event for the audio track, with a stream that only contains the audio track, then another `track` event for the video track, where the video track has now been added to the stream.

But it seems more sensible to fire the `track` event with the full list of tracks already populated. 

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1135 using your GitHub account