[webrtc-pc] Pull Request: Ensure that "track" events fire before the SRD promise resolves.

taylor-b has just submitted a new pull request for https://github.com/w3c/webrtc-pc:

== Ensure that "track" events fire before the SRD promise resolves. ==
Fixes #1508.

This was the behavior before, but I accidentally broke it in PR #1136.

This ensures that if, for example, a description is applied with
audio/video tracks in the same stream, the order of events is:

1. "track" event with audio track, and stream that contains both tracks.
2. "track" event with video track, and stream that contains both tracks.
3. SRD promise resolves.

See https://github.com/w3c/webrtc-pc/pull/1519