[ObjC ARC] Converts ios/web/shell/test to ARC.

ios/web/shell/test/app/web_shell_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/web/shell/test/app/web_shell_test_util.h"
 
 #import <UIKit/UIKit.h>
 
 #import "ios/web/shell/view_controller.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 namespace shell_test_util {
 
 web::WebState* GetCurrentWebState() {
   ViewController* view_controller = static_cast<ViewController*>([[
       [[UIApplication sharedApplication] delegate] window] rootViewController]);
   return view_controller.webState;
 }
 
 }  // namespace shell_test_util
 }  // namespace web