[webrtc-pc] Spec note on stop() underestimates BUNDLE problem. (#2176)

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

== Spec note on stop() underestimates BUNDLE problem. ==
Our [BUNDLE warning](http://w3c.github.io/webrtc-pc/#methods-8) says:
> Note
> 
> If this method is called in between applying a remote offer and creating an answer, and the transceiver is associated with the "offerer tagged" media description as defined in [BUNDLE], this will cause all other transceivers in the bundle group to be stopped as well.

This suggests the problem to limited calling `stop()` in `"have-remote-offer"`, but the same problem exists if `stop()` has been called *before* SRD(offer).

> To avoid this, one could instead stop the transceiver when signalingState is "stable" and perform a subsequent offer/answer exchange.

This is true only if we replace "a subsequent" with "an immediate", and assume an SRD(offer) cannot come in beforehand. It also assumes our offer isn't rolled back.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2176 using your GitHub account