Bug 1468451: Add a closed check to addTransceiver. r+jib r=jib

MozReview-Commit-ID: DMmorKFz5EL

Differential Revision: https://phabricator.services.mozilla.com/D6280

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

dom/media/PeerConnection.js

@@ -1356,6 +1356,7 @@ class RTCPeerConnection {
   }
 
   addTransceiver(sendTrackOrKind, init) {
+    this._checkClosed();
     let transceiver = this._addTransceiverNoEvents(sendTrackOrKind, init);
     this.updateNegotiationNeeded();
     return transceiver;