Use variadic template for screen handler's AddCallback.

components/login/base_screen_handler_utils.cc

 // Copyright 2014 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 "components/login/base_screen_handler_utils.h"
 
 namespace login {
 
 namespace {
 
@@ skipping 58 matching lines @@
 }
 
 base::StringValue MakeValue(const std::string& v) {
   return base::StringValue(v);
 }
 
 base::StringValue MakeValue(const base::string16& v) {
   return base::StringValue(v);
 }
 
-void CallbackWrapper0(base::Callback<void()> callback,
-                      const base::ListValue* args) {
-  DCHECK(args);
-  DCHECK(args->empty());
-  callback.Run();
-}
-
 }  // namespace login