Tries to enable FEC for opus.

Sets useinbandfec=1 in everyone's remoteDescription, which should be
enough to enable the use of FEC with chrome versions which support it
and when there is packet loss.

See:
https://code.google.com/p/webrtc/issues/detail?id=3986
https://code.google.com/p/webrtc/issues/detail?id=2419#c27

I don't know of an easy way to verify that this works, but I think it's a safe thing to do (apprtc seems to the the same if the opusfec=true parameter is given).