Refactor the way singleton-style resources are exposed via PPB_Instance

ppapi/proxy/plugin_dispatcher.h

Index: ppapi/proxy/plugin_dispatcher.h
diff --git a/ppapi/proxy/plugin_dispatcher.h b/ppapi/proxy/plugin_dispatcher.h
index a6cc72048fa62992407a4720aa26ad5f97e65c20..c66f08cce695bd9ab855149b0e106be0a36a27fa 100644
--- a/ppapi/proxy/plugin_dispatcher.h
+++ b/ppapi/proxy/plugin_dispatcher.h
@@ -51,11 +51,10 @@ struct InstanceData {
   // When non-NULL, indicates the callback to execute when mouse lock is lost.
   scoped_refptr<TrackedCallback> mouse_lock_callback;
 
-  // The following are lazily created the first time the plugin requests them.
-  // (These are singleton-style resources).
-  scoped_refptr<GamepadResource> gamepad_resource;
-  scoped_refptr<FlashResource> flash_resource;
-  scoped_refptr<FlashClipboardResource> flash_clipboard_resource;
+  // A map of singleton resources which are lazily created.
+  typedef std::map<SingletonResourceID, scoped_refptr<Resource> >
+      SingletonResourceMap;
+  SingletonResourceMap singleton_resources;
 
   // Calls to |RequestSurroundingText()| are done by posted tasks. Track whether
   // a) a task is pending, to avoid redundant calls, and b) whether we should