Include msid-semantic in Session Attributes

Chrome doesn't include the msid when responding to
recvonly transceivers

See crbug[0] for discussion

[0] https://issues.chromium.org/u/1/issues/328522463

peerconnection.go

@@ -2318,6 +2318,7 @@ func (pc *PeerConnection) generateUnmatchedSDP(transceivers []*RTPTransceiver, u
 	if err != nil {
 		return nil, err
 	}
+	d.Attributes = append(d.Attributes, sdp.Attribute{Key: sdp.AttrKeyMsidSemantic, Value: "WMS*"})
 
 	iceParams, err := pc.iceGatherer.GetLocalParameters()
 	if err != nil {
@@ -2390,6 +2391,7 @@ func (pc *PeerConnection) generateMatchedSDP(transceivers []*RTPTransceiver, use
 	if err != nil {
 		return nil, err
 	}
+	d.Attributes = append(d.Attributes, sdp.Attribute{Key: sdp.AttrKeyMsidSemantic, Value: "WMS*"})
 
 	iceParams, err := pc.iceGatherer.GetLocalParameters()
 	if err != nil {