Refactor the way singleton-style resources are exposed via PPB_Instance

ppapi/thunk/ppb_gamepad_thunk.cc

Index: ppapi/thunk/ppb_gamepad_thunk.cc
diff --git a/ppapi/thunk/ppb_gamepad_thunk.cc b/ppapi/thunk/ppb_gamepad_thunk.cc
index 3aeedb57dea12d116e6b0f112e98327f5ff1510b..de2c8be04cf54f30dcd3ed82d82087d23f3e0abf 100644
--- a/ppapi/thunk/ppb_gamepad_thunk.cc
+++ b/ppapi/thunk/ppb_gamepad_thunk.cc
@@ -17,13 +17,10 @@ namespace thunk {
 namespace {
 
 void SampleGamepads(PP_Instance instance, PP_GamepadsSampleData* data) {
-  EnterInstance enter(instance);
+  EnterInstanceAPI<PPB_Gamepad_API> enter(instance);
   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));