Interfaceify ClientPlugin in preparation for mocking it.

remoting/webapp/remoting.js

Index: remoting/webapp/remoting.js
diff --git a/remoting/webapp/remoting.js b/remoting/webapp/remoting.js
index fc70aa4483c101bb97f2e5a96060b3920d885afd..4e34d50025ce4c1988512f7e03bf341babfbab04 100644
--- a/remoting/webapp/remoting.js
+++ b/remoting/webapp/remoting.js
@@ -52,6 +52,9 @@ remoting.init = function() {
   l10n.localize();
 
   // Create global objects.
+  remoting.ClientPlugin.factory = new remoting.DefaultClientPluginFactory();
+  remoting.SessionConnector.factory =
+      new remoting.DefaultSessionConnectorFactory();
   remoting.settings = new remoting.Settings();
   if (base.isAppsV2()) {
     remoting.identity = new remoting.Identity(consentRequired_);
@@ -195,7 +198,7 @@ remoting.init = function() {
   };
   remoting.testEvents.defineEvents(base.values(remoting.testEvents.Names));
 
-  remoting.ClientPlugin.preload();
+  remoting.ClientPlugin.factory.preloadPlugin();
 };
 
 /**