Move ProtocolExtensionManager from SessionConnector into its own class

remoting/webapp/crd/js/client_plugin.js

Index: remoting/webapp/crd/js/client_plugin.js
diff --git a/remoting/webapp/crd/js/client_plugin.js b/remoting/webapp/crd/js/client_plugin.js
index 325117f51713e10483b74356dfe00193b631dc1a..e8b3f8d9a9525ebfa3672b8141d52e50198e3c38 100644
--- a/remoting/webapp/crd/js/client_plugin.js
+++ b/remoting/webapp/crd/js/client_plugin.js
@@ -24,6 +24,11 @@ remoting.ClientPlugin = function() {};
 remoting.ClientPlugin.prototype.hostDesktop = function() {};
 
 /**
+ * @return {remoting.ProtocolExtensionManager}
+ */
+remoting.ClientPlugin.prototype.extensions = function() {};
+
+/**
  * @return {HTMLElement} The DOM element representing the remote session.
  */
 remoting.ClientPlugin.prototype.element = function() {};
@@ -231,14 +236,6 @@ remoting.ClientPlugin.ConnectionEventHandler.prototype.onSetCapabilities =
     function(capabilities) {};
 
 /**
- * @param {string} type
- * @param {string} data
- */
-remoting.ClientPlugin.ConnectionEventHandler.prototype.onExtensionMessage =
-    function(type, data) {};
-
-
-/**
  * @interface
  */
 remoting.ClientPluginFactory = function() {};