Refactor the way singleton-style resources are exposed via PPB_Instance

ppapi/thunk/ppb_gamepad_thunk.cc

 // 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.
 
 #include <string.h>
 
 #include "ppapi/c/ppb_gamepad.h"
 #include "ppapi/thunk/thunk.h"
 #include "ppapi/thunk/enter.h"
 #include "ppapi/thunk/ppb_gamepad_api.h"
 #include "ppapi/thunk/ppb_instance_api.h"
 #include "ppapi/thunk/resource_creation_api.h"
 
 namespace ppapi {
 namespace thunk {
 
 namespace {
 
 void SampleGamepads(PP_Instance instance, PP_GamepadsSampleData* data) {
-  EnterInstance enter(instance);
+  EnterInstanceAPI<PPB_Gamepad_API> enter(instance, GAMEPAD_SINGLETON_ID);
   if (enter.succeeded()) {
-    PPB_Gamepad_API* api = enter.functions()->GetGamepadAPI(instance);
-    if (api) {
-      api->Sample(data);
-      return;
-    }
+    enter.functions()->Sample(data);
+    return;
   }
   // Failure, zero out.
   memset(data, 0, sizeof(PP_GamepadsSampleData));
 }
 
 const PPB_Gamepad g_ppb_gamepad_thunk = {
   &SampleGamepads,
 };
 
 }  // namespace
 
 const PPB_Gamepad* GetPPB_Gamepad_1_0_Thunk() {
   return &g_ppb_gamepad_thunk;
 }
 
 }  // namespace thunk
 }  // namespace ppapi