code: function addStreamToPeerConnection(stream, peerConnection) {
         if( peerConnection.addStream ) {
             var existingStreams = peerConnection.getLocalStreams();
             if (existingStreams.indexOf(stream) === -1) {
                peerConnection.addStream(stream);
             }
         }
         else {
            var existingTracks = peerConnection.getSenders();
            var tracks = stream.getAudioTracks();
            var i;
            for( i = 0; i < tracks.length; i++ ) {
              if (existingTracks.indexOf(tracks[i]) === -1) {
               peerConnection.addTrack( tracks[i]);
              }
            }
            tracks = stream.getVideoTracks();
            for( i = 0; i < tracks.length; i++ ) {
              if (existingTracks.indexOf(tracks[i]) === -1) {
               peerConnection.addTrack( tracks[i]);
              }
            }
         }
      }
docs: this private method handles adding a stream to a peer connection. chrome only supports adding streams, safari only supports adding tracks.