[webrtc-pc] new commits pushed by aboba

The following commits were just pushed by aboba to https://github.com/w3c/webrtc-pc:

* Block replaceTrack on pc.close() like the others, not tc.stop().
  by Jan-Ivar Bruaroey
https://github.com/w3c/webrtc-pc/commit/7ab0e11f061e4884243233d60c406ebf0b053796

* Merge pull request #2107 from jan-ivar/isclosed

Block replaceTrack on pc.close() like the others, not tc.stop().
  by Bernard Aboba
https://github.com/w3c/webrtc-pc/commit/0e1f5b95aa40284410b3d00d8bc9f477c8b15e36