feat(ssrc-rewriting) Advertise support to Jicofo. (#2209)

* feat(ssrc-rewriting) Advertise support to Jicofo.

* Fix unit test

modules/xmpp/SignalingLayerImpl.spec.js

                 );
             });
             it('from a user with SourceInfo and ssrc-rewriting disabled', () => {
+                FeatureFlags.init({ });
                 const emitterSpy = spyOn(signalingLayer.eventEmitter, 'emit');
                 const sourceInfo = {
                     '12345678-a0': {

modules/xmpp/xmpp.js

 import { XMPPEvents } from '../../service/xmpp/XMPPEvents';
 import browser from '../browser';
 import { E2EEncryption } from '../e2ee/E2EEncryption';
+import FeatureFlags from '../flags/FeatureFlags';
 import Statistics from '../statistics/statistics';
 import GlobalOnErrorHandler from '../util/GlobalOnErrorHandler';
 import Listenable from '../util/Listenable';
 
         logger.debug('Receiving multiple video streams is enabled');
         this.caps.addFeature('http://jitsi.org/receive-multiple-video-streams');
+
+        // Advertise support for ssrc-rewriting.
+        if (FeatureFlags.isSsrcRewritingSupported()) {
+            this.caps.addFeature('http://jitsi.org/ssrc-rewriting-1');
+        }
     }
 
     /**