[ObjC ARC] Converts ios/chrome/browser/signin:signin to ARC.

ios/chrome/browser/signin/authentication_service_factory.mm

 // Copyright 2013 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.
 
 #import "ios/chrome/browser/signin/authentication_service_factory.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/memory/singleton.h"
 #include "components/keyed_service/ios/browser_state_dependency_manager.h"
 #include "ios/chrome/browser/browser_state/chrome_browser_state.h"
 #include "ios/chrome/browser/signin/account_tracker_service_factory.h"
 #import "ios/chrome/browser/signin/authentication_service.h"
 #include "ios/chrome/browser/signin/oauth2_token_service_factory.h"
 #include "ios/chrome/browser/signin/signin_manager_factory.h"
 #include "ios/chrome/browser/sync/sync_setup_service_factory.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 // static
 AuthenticationService* AuthenticationServiceFactory::GetForBrowserState(
     ios::ChromeBrowserState* browser_state) {
   return static_cast<AuthenticationService*>(
       GetInstance()->GetServiceForBrowserState(browser_state, true));
 }
 
 AuthenticationService* AuthenticationServiceFactory::GetForBrowserStateIfExists(
     ios::ChromeBrowserState* browser_state) {
   return static_cast<AuthenticationService*>(
@@ skipping 30 matching lines @@
       SyncSetupServiceFactory::GetForBrowserState(browser_state)));
   service->Initialize();
   // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(service);
 }
 
 void AuthenticationServiceFactory::RegisterBrowserStatePrefs(
     user_prefs::PrefRegistrySyncable* registry) {
   AuthenticationService::RegisterPrefs(registry);
 }