Refactor the way singleton-style resources are exposed via PPB_Instance

ppapi/proxy/gamepad_resource.cc

Index: ppapi/proxy/gamepad_resource.cc
diff --git a/ppapi/proxy/gamepad_resource.cc b/ppapi/proxy/gamepad_resource.cc
index 8a4943767cc30b9fff1c26141050a8a868dc0cad..8bbf8cc6c28cf3c124db5e186bcd2b22f8d1b7d6 100644
--- a/ppapi/proxy/gamepad_resource.cc
+++ b/ppapi/proxy/gamepad_resource.cc
@@ -58,6 +58,10 @@ GamepadResource::GamepadResource(Connection connection, PP_Instance instance)
 GamepadResource::~GamepadResource() {
 }
 
+thunk::PPB_Gamepad_API* GamepadResource::AsPPB_Gamepad_API() {
+  return this;
+}
+
 void GamepadResource::Sample(PP_GamepadsSampleData* data) {
   if (!buffer_) {
     // Browser hasn't sent back our shared memory, give the plugin gamepad