Re: [webrtc-pc] "a=msid" line should contain sender/receiver IDs, not track IDs

```
senderA = pc.addTrack(track);  // a=msid with track.id
O/A
senderA.replaceTrack(null or something else);
senderB = pc.addTrack(track);  // a=msid with...?
```
This will end up on a different m-line / transceiver.

> pc.addTransceiver('video'); // track is null by default, a=msid with...?

http://rtcweb-wg.github.io/jsep/#rfc.section.5.2.1 -- If no MediaStreamTrack is attached, a valid ID MUST be generated, in the same way that the implementation generates IDs for local tracks.


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