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

ios/chrome/browser/ui/util/top_view_controller.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/browser/ui/util/top_view_controller.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace top_view_controller {
 
 UIViewController* TopPresentedViewController() {
   return TopPresentedViewControllerFrom(
       [[UIApplication sharedApplication] keyWindow].rootViewController);
 }
 UIViewController* TopPresentedViewControllerFrom(
     UIViewController* base_view_controller) {
   UIViewController* topController = base_view_controller;
   for (UIViewController* controller = [topController presentedViewController];
        controller && ![controller isBeingDismissed];
        controller = [controller presentedViewController]) {
     topController = controller;
   }
   return topController;
 }
 
 }  // namespace top_view_controller