webrtc-pc

f84a16fd - Clarify when removeTrack should throw an exception.

Clarify when removeTrack should throw an exception. In PR #844, the intent was to throw an exception when the sender wasn't created by the connection on which the method was called. And the assumption was that senders are never removed, so the algorithm can simply check if it's in the connection's set of senders. But something I didn't consider is that rollbacks are able to remove transceivers (and thus senders). So more clarity is needed.