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

ios/chrome/test/app/stack_view_test_util.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/test/app/stack_view_test_util.h"
 
 #import "base/mac/foundation_util.h"
 #import "ios/chrome/app/main_controller.h"
 #import "ios/chrome/app/main_controller_private.h"
 #import "ios/chrome/browser/ui/stack_view/stack_view_controller.h"
 #import "ios/chrome/test/app/chrome_test_util.h"
 #import "ios/chrome/test/app/tab_test_util.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace chrome_test_util {
 
 bool IsTabSwitcherActive() {
   return [chrome_test_util::GetMainController() isTabSwitcherActive];
 }
 
 StackViewController* GetStackViewController() {
   if (!IsTabSwitcherActive())
     return nil;
   MainController* mainController = chrome_test_util::GetMainController();
   DCHECK(mainController);
   return base::mac::ObjCCastStrict<StackViewController>(
       [mainController tabSwitcherController]);
 }
 
 }  // namespace chrome_test_util