[ObjC ARC] Converts ios/chrome/app:tests_fake_hook to ARC.

ios/chrome/app/tests_fake_hook.mm

 // Copyright 2016 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/app/tests_hook.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace tests_hook {
 
 bool DisableContextualSearch() {
   return false;
 }
 bool DisableFirstRun() {
   return false;
 }
 bool DisableGeolocation() {
   return false;
 }
 bool DisableSigninRecallPromo() {
   return false;
 }
 bool DisableUpdateService() {
   return false;
 }
 void SetUpTestsIfPresent() {}
 void RunTestsIfPresent() {}
 
 }  // namespace tests_hook