Refactor the way singleton-style resources are exposed via PPB_Instance

ppapi/thunk/ppb_gamepad_api.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PPAPI_THUNK_PPB_GAMEPAD_API_H_
 #define PPAPI_THUNK_PPB_GAMEPAD_API_H_
 
+#include "ppapi/shared_impl/singleton_resource_id.h"
 #include "ppapi/thunk/ppapi_thunk_export.h"
 
 struct PP_GamepadsSampleData;
 
 namespace ppapi {
 namespace thunk {
 
 class PPAPI_THUNK_EXPORT PPB_Gamepad_API {
  public:
   virtual ~PPB_Gamepad_API() {}
 
   virtual void Sample(PP_GamepadsSampleData* data) = 0;
+
+  static const SingletonResourceID kSingletonResourceID = GAMEPAD_SINGLETON_ID;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_GAMEPAD_API_H_