ref(JingleSessionPC): tear down remote streams state

Reset the state of remote MediaStreams in order to fix sequence number
synchronization problem. When a conference is moved to another bridge or
if channels are re-allocated the bridge looses the most recent sequence
number seen by the client which makes Chrome drop all video packets
until the JVB catches up with the sequence numbers (if at all).

modules/xmpp/JingleSessionPC.js

@@ -988,6 +988,19 @@ export default class JingleSessionPC extends JingleSession {
             .find('>content[name=\'data\']')
             .attr('senders', 'rejected');
 
+        // Remove all remote sources in order to reset the client's state
+        // for the remote MediaStreams. When a conference is moved to
+        // another bridge it will start streaming with a sequence number
+        // that is not in sync with the most recently seen by the client.
+        // The symptoms include frozen or black video and lots of "failed to
+        // unprotect SRTP packets" in Chrome logs.
+        jingleOfferElem
+            .find('>content>description>source')
+            .remove();
+        jingleOfferElem
+            .find('>content>description>ssrc-group')
+            .remove();
+
         // First set an offer with a rejected 'data' section
         this.setOfferAnswerCycle(
             jingleOfferElem,