Bug 1595479 - Call Stop as part of Transceiver shutdown; r=ng

This consolidates the shutdown code and ensures that an RTCP bye is sent
as part of tearing down a PeerConnection.

Differential Revision: https://phabricator.services.mozilla.com/D57861

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

media/webrtc/signaling/src/peerconnection/TransceiverImpl.cpp

@@ -118,17 +118,12 @@ nsresult TransceiverImpl::UpdateSinkIdentity(
 }
 
 void TransceiverImpl::Shutdown_m() {
-  mReceivePipeline->Shutdown_m();
-  mTransmitPipeline->Shutdown_m();
+  Stop();
   mReceivePipeline = nullptr;
   mTransmitPipeline = nullptr;
   mTransportHandler = nullptr;
   mReceiveTrack = nullptr;
   mSendTrack = nullptr;
-  if (mConduit) {
-    mConduit->DeleteStreams();
-  }
-  mConduit = nullptr;
 }
 
 nsresult TransceiverImpl::UpdateSendTrack(dom::MediaStreamTrack* aSendTrack) {