Bug 1551854: Don't stomp the mid on a transceiver if the answer has n…

…o mid in it. r=mjf

Depends on D37076

Differential Revision: https://phabricator.services.mozilla.com/D37077

--HG--
extra : moz-landing-system : lando

media/webrtc/signaling/src/jsep/JsepSessionImpl.cpp

@@ -1446,7 +1446,9 @@ nsresult JsepSessionImpl::UpdateTransceiversFromRemoteDescription(
     }
 
     if (!mSdpHelper.MsectionIsDisabled(msection)) {
-      transceiver->Associate(msection.GetAttributeList().GetMid());
+      if (msection.GetAttributeList().HasAttribute(SdpAttribute::kMidAttribute)) {
+        transceiver->Associate(msection.GetAttributeList().GetMid());
+      }
       if (!transceiver->IsAssociated()) {
         transceiver->Associate(GetNewMid());
       } else {