Bug 1528323 - Part 1.1: Fix bug in _queueWithClosedCheck where |this| did not refer to the PeerConnection. r=jib

Differential Revision: https://phabricator.services.mozilla.com/D21015

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

dom/media/PeerConnection.jsm

@@ -586,10 +586,11 @@ class RTCPeerConnection {
 
   // This implements the fairly common "Queue a task" logic
   async _queueTaskWithClosedCheck(func) {
+    const pc = this;
     return new this._win.Promise((resolve, reject) => {
       Services.tm.dispatchToMainThread({ run() {
         try {
-          if (!this._closed) {
+          if (!pc._closed) {
             func();
             resolve();
           }