Re: [webrtc-pc] No way to add track with stream associations to a given transceiver

I propose:
```js
transceiver.sender.setStreams([cameraStream]);
```
We could either:
 1. have it throw outside `"have-remote-offer"` or if the transceiver is used, to limit scope, or
 2. explore letting users change existing associations this way as well (have it trigger negotiationneeded in `"stable"` state).

I seem to recall someone asking for the latter not too long ago. One less reason to mangle SPD, which we'll likely [end up supporting](https://github.com/w3c/webrtc-pc/issues/1729#issuecomment-356787787) on the far end anyway.

-- 
GitHub Notification of comment by jan-ivar
Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1826#issuecomment-378808993 using your GitHub account