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

ios/chrome/browser/ui/page_not_available_controller.h

 // Copyright 2012 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.
 
 #ifndef IOS_CHROME_BROWSER_UI_PAGE_NOT_AVAILABLE_CONTROLLER_H_
 #define IOS_CHROME_BROWSER_UI_PAGE_NOT_AVAILABLE_CONTROLLER_H_
 
 #import "ios/chrome/browser/ui/native_content_controller.h"
 
 // A native controller for the case where the URL is not valid and/or the page
 // is not available.
 @interface PageNotAvailableController : NativeContentController
 
 // Property to allow setting a custom description for the content.
-@property(nonatomic, assign) NSString* descriptionText;
+@property(nonatomic, copy) NSString* descriptionText;
 
 // Designated initializer.
 - (instancetype)initWithUrl:(const GURL&)url NS_DESIGNATED_INITIALIZER;
 
 - (instancetype)initWithNibName:(NSString*)nibName
                             url:(const GURL&)url NS_UNAVAILABLE;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_PAGE_NOT_AVAILABLE_CONTROLLER_H_