Move ProtocolExtensionManager from SessionConnector into its own class

remoting/webapp/crd/js/desktop_remoting.js

Index: remoting/webapp/crd/js/desktop_remoting.js
diff --git a/remoting/webapp/crd/js/desktop_remoting.js b/remoting/webapp/crd/js/desktop_remoting.js
index 9c3156fdf9dd43292d83a64125f71cdf1c8489ca..62a42b861265f3890122cdd5a9c69d640075d1df 100644
--- a/remoting/webapp/crd/js/desktop_remoting.js
+++ b/remoting/webapp/crd/js/desktop_remoting.js
@@ -183,18 +183,17 @@ remoting.DesktopRemoting.prototype.onConnected_ = function(connectionInfo) {
     connectionInfo.plugin().setRemapKeys('0x0700e4>0x0700e7');
   }
 
+  var plugin = connectionInfo.plugin();
   if (remoting.app.getConnectionMode() === remoting.Application.Mode.ME2ME) {
     if (remoting.app.hasCapability(remoting.ClientSession.Capability.CAST)) {
-      this.sessionConnector_.registerProtocolExtension(
-          new remoting.CastExtensionHandler());
+      plugin.extensions().register(new remoting.CastExtensionHandler());
     }
-    this.sessionConnector_.registerProtocolExtension(
-        new remoting.GnubbyAuthHandler());
+    plugin.extensions().register(new remoting.GnubbyAuthHandler());
   }
   if (connectionInfo.session().hasCapability(
           remoting.ClientSession.Capability.VIDEO_RECORDER)) {
     var recorder = new remoting.VideoFrameRecorder();
-    this.sessionConnector_.registerProtocolExtension(recorder);
+    plugin.extensions().register(recorder);
     this.connectedView_.setVideoFrameRecorder(recorder);
   }