strip all rtx if we're using unified plan (since sdp-interop doesn't

necessarily handle the translation of groups correctly)

modules/xmpp/RtxModifier.js

           logger.info("RtxModifier doing nothing, no video ssrcs present");
           return sdpStr;
         }
+        if (!videoMLine.ssrcGroups) {
+          logger.info("RtxModifier doing nothing, no video ssrcGroups present");
+          return sdpStr;
+        }
         const fidGroups = videoMLine.ssrcGroups
             .filter(group => group.semantics === "FID");
         // Remove the fid groups from the mline

modules/xmpp/RtxModifier.spec.js

 
     describe("stripRtx", function() {
         beforeEach(function() {
-            this.sdpStr = transform.write(SampleSdpStrings.rtxVideoSdp);
         });
         it ("should strip all rtx streams from an sdp with rtx", function() {
-            const newSdpStr = this.rtxModifier.stripRtx(this.sdpStr);
+            const sdpStr = transform.write(SampleSdpStrings.rtxVideoSdp);
+            const newSdpStr = this.rtxModifier.stripRtx(sdpStr);
             const newSdp = transform.parse(newSdpStr);
             const fidGroups = getVideoGroups(newSdp, "FID");
             expect(fidGroups.length).toEqual(0);
-            const videoMLine = SDPUtil.getMedia(newSdp, "video");
-            expect(videoMLine.ssrcs.length).toEqual(1);
-
+            expect(numVideoSsrcs(newSdp)).toEqual(1);
+        });
+        it ("should do nothing to an sdp with no rtx", function() {
+            const sdpStr = transform.write(SampleSdpStrings.plainVideoSdp);
+            const newSdpStr = this.rtxModifier.stripRtx(sdpStr);
+            expect(newSdpStr).toEqual(sdpStr);
         });
     });
 });

modules/xmpp/TraceablePeerConnection.js

     description = this.simulcast.mungeRemoteDescription(description);
     this.trace('setRemoteDescription::postTransform (simulcast)', dumpSDP(description));
 
-    //description.sdp = this.rtxModifier.implodeRemoteRtxSsrcs(description.sdp);
-    //this.trace('setRemoteDescription::postTransform (implodeRemoteRtxSsrcs)', dumpSDP(description));
-
     // if we're running on FF, transform to Plan A first.
     if (RTCBrowserType.usesUnifiedPlan()) {
+        description.sdp = this.rtxModifier.stripRtx(description.sdp);
+        this.trace('setRemoteDescription::postTransform (stripRtx)', dumpSDP(description));
         description = this.interop.toUnifiedPlan(description);
         this.trace('setRemoteDescription::postTransform (Plan A)', dumpSDP(description));
     }