[webrtc-pc] is addTrack(track, stream, stream) allowed? (#2803)

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

== is addTrack(track, stream, stream) allowed? ==
This came up while looking at how Chrome parses msid:

https://jsfiddle.net/fippo/y2xso6Lq/1/
results in
* two identical a=msid lines being generated
* In Chrome ontrack being called with two MediaStreams which are not identical. In Firefox only one MediaStream is generated

My interpretation of https://www.rfc-editor.org/rfc/rfc8830.html#section-3.2.2 is that the parsing is not valid.

https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-addtrack
has a step saying
For each stream in streams, add stream.id to [[[AssociatedMediaStreamIds]]](https://w3c.github.io/webrtc-pc/#dfn-associatedmediastreamids) *if it's not already there*

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2803 using your GitHub account


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