Bug 1435013: Sync transceivers before creating offers/answers. r=jib

MozReview-Commit-ID: EORXVc8Hnky

--HG--
extra : rebase_source : 1ea186ac38f44b83f247b51224dd83be0b09df42

dom/media/PeerConnection.js

@@ -847,6 +847,7 @@ class RTCPeerConnection {
 
   async _createOffer(options) {
     this._checkClosed();
+    this._syncTransceivers();
     let origin = Cu.getWebIDLCallerPrincipal().origin;
     return this._chain(async () => {
       let haveAssertion;
@@ -878,6 +879,7 @@ class RTCPeerConnection {
 
   async _createAnswer(options) {
     this._checkClosed();
+    this._syncTransceivers();
     let origin = Cu.getWebIDLCallerPrincipal().origin;
     return this._chain(async () => {
       // We give up line-numbers in errors by doing this here, but do all