Refactor the way singleton-style resources are exposed via PPB_Instance

ppapi/thunk/ppb_gamepad_api.h

Index: ppapi/thunk/ppb_gamepad_api.h
diff --git a/ppapi/thunk/ppb_gamepad_api.h b/ppapi/thunk/ppb_gamepad_api.h
index ee7199a5d801ed82656675babf97cabd933124c4..2759831021b85eeb7b35545f291d9c1d4bcdba7d 100644
--- a/ppapi/thunk/ppb_gamepad_api.h
+++ b/ppapi/thunk/ppb_gamepad_api.h
@@ -5,6 +5,7 @@
 #ifndef PPAPI_THUNK_PPB_GAMEPAD_API_H_
 #define PPAPI_THUNK_PPB_GAMEPAD_API_H_
 
+#include "ppapi/shared_impl/api_id.h"
 #include "ppapi/thunk/ppapi_thunk_export.h"
 
 struct PP_GamepadsSampleData;
@@ -17,6 +18,8 @@ class PPAPI_THUNK_EXPORT PPB_Gamepad_API {
   virtual ~PPB_Gamepad_API() {}
 
   virtual void Sample(PP_GamepadsSampleData* data) = 0;
+
+  static const SingletonResourceID kSingletonResourceID = GAMEPAD_SINGLETON_ID;
 };
 
 }  // namespace thunk