Bug 1604746 - Recreate the send track in the MTG of the provided track if they belong to different MTGs. r=padenot

In `MediaPipelineTransmit::SetTrack()` the provided `MediaStreamTrack` connects to the existing send-track. This step crashes if the two tracks belong to different MTGs. With this change, when the two tracks belong to different MTGs, the existing send-track is stopped and deleted and a new send-track is created in the same MTG with the provided `MediaStreamTrack`.

Differential Revision: https://phabricator.services.mozilla.com/D57627

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

media/webrtc/signaling/src/mediapipeline/MediaPipeline.cpp

@@ -1003,13 +1003,24 @@ nsresult MediaPipelineTransmit::SetTrack(RefPtr<MediaStreamTrack> aDomTrack) {
     mSendPort = nullptr;
   }
 
+  bool wasTransmitting = false;
+  if (aDomTrack && mDomTrack && !aDomTrack->Ended() && !mDomTrack->Ended() &&
+      aDomTrack->Graph() != mDomTrack->Graph() && mSendTrack) {
+    // Recreate the send track if the new stream resides in different MTG.
+    wasTransmitting = mTransmitting;
+    Stop();
+    mSendTrack->Destroy();
+    mSendTrack = nullptr;
+  }
+
   mDomTrack = std::move(aDomTrack);
   SetDescription();
 
   if (mDomTrack) {
     if (!mDomTrack->Ended()) {
       if (!mSendTrack) {
-        // Create the send track only once; when the first live track is set.
+        // Create the send track when the first live track is set or when the
+        // new track resides in different MTG.
         SetSendTrack(mDomTrack->Graph()->CreateForwardedInputTrack(
             mDomTrack->GetTrack()->mType));
       }
@@ -1020,6 +1031,9 @@ nsresult MediaPipelineTransmit::SetTrack(RefPtr<MediaStreamTrack> aDomTrack) {
       mConverter->SetTrackEnabled(mDomTrack->Enabled());
     }
   }
+  if (wasTransmitting) {
+    Start();
+  }
 
   return NS_OK;
 }