Backed out changeset dba7a059ed22 (bug 843214)
Backed out changeset 05c35dc73323 (bug 845526)
Backed out changeset 622b744e5df9 (bug 829557)
Backed out changeset f73b7b8d1a2d (bug 829557) for failing to compile on Windows on a CLOSED TREE
Bug 843214. Make SourceMediaStream::AddTrack/AppendToTrack/HaveEnoughBuffered/DispatchWhenNotEnoughBuffered/EndTrack smoothly handle cases where track does not exist. r=jesup