fix(TPC): Implode the simulcast group only after toUnifiedPlan conversion.

This fixes a regression introduced by the previous commit.

modules/RTC/TraceablePeerConnection.js

@@ -2374,15 +2374,6 @@ TraceablePeerConnection.prototype.setRemoteDescription = function(description) {
     // Munge stereo flag and opusMaxAverageBitrate based on config.js
     remoteDescription = this._mungeOpus(remoteDescription);
 
-    if (this.isSimulcastOn()) {
-        // Add x-google-conference flag for plan-b mode.
-        const addGoogConfFlag = (browser.isChromiumBased() || browser.isReactNative()) && !this._usesUnifiedPlan;
-
-        // Implode the simulcast ssrcs so that the remote sdp has only the first ssrc in the SIM group.
-        remoteDescription = this.simulcast.mungeRemoteDescription(remoteDescription, addGoogConfFlag);
-        this.trace('setRemoteDescription::postTransform (simulcast)', dumpSDP(remoteDescription));
-    }
-
     if (this._usesUnifiedPlan) {
         // Translate the SDP to Unified plan format first for the jvb case, p2p case will only have 2 m-lines.
         if (!this.isP2P) {
@@ -2392,17 +2383,29 @@ TraceablePeerConnection.prototype.setRemoteDescription = function(description) {
             this.trace('setRemoteDescription::postTransform (Unified)', dumpSDP(remoteDescription));
         }
         if (this.isSimulcastOn()) {
+            // Implode the simulcast ssrcs so that the remote sdp has only the first ssrc in the SIM group.
+            remoteDescription = this.simulcast.mungeRemoteDescription(remoteDescription);
+            this.trace('setRemoteDescription::postTransform (simulcast)', dumpSDP(remoteDescription));
+
             remoteDescription = this.tpcUtils.insertUnifiedPlanSimulcastReceive(remoteDescription);
             this.trace('setRemoteDescription::postTransform (sim receive)', dumpSDP(remoteDescription));
         }
         remoteDescription = this.tpcUtils.ensureCorrectOrderOfSsrcs(remoteDescription);
         this.trace('setRemoteDescription::postTransform (correct ssrc order)', dumpSDP(remoteDescription));
     } else {
+        if (this.isSimulcastOn()) {
+            // Implode the simulcast ssrcs so that the remote sdp has only the first ssrc in the SIM group.
+            remoteDescription = this.simulcast.mungeRemoteDescription(
+                remoteDescription,
+                true /* add x-google-conference flag */);
+            this.trace('setRemoteDescription::postTransform (simulcast)', dumpSDP(remoteDescription));
+        }
         remoteDescription = normalizePlanB(remoteDescription);
     }
 
     // Munge the order of the codecs based on the preferences set through config.js.
     remoteDescription = this._mungeCodecOrder(remoteDescription);
+    this.trace('setRemoteDescription::postTransform (munge codec order)', dumpSDP(remoteDescription));
 
     return new Promise((resolve, reject) => {
         this.peerconnection.setRemoteDescription(remoteDescription)