Convert ios_web_shell to ARC.

ios/web/shell/view_controller.h

 // Copyright 2014 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_WEB_SHELL_VIEW_CONTROLLER_H_
 #define IOS_WEB_SHELL_VIEW_CONTROLLER_H_
 
 #import <UIKit/UIKit.h>
 
 namespace web {
 class BrowserState;
 class WebState;
 }
 
 // Accessibility label for the back button.
 extern NSString* const kWebShellBackButtonAccessibilityLabel;
 // Accessibility label for the forward button.
 extern NSString* const kWebShellForwardButtonAccessibilityLabel;
 // Accessibility label for the URL address text field.
 extern NSString* const kWebShellAddressFieldAccessibilityLabel;
 
 // Implements the main UI for ios_web_shell, including a toolbar and web view.
 @interface ViewController : UIViewController
 
-@property(nonatomic, retain) IBOutlet UIView* containerView;
-@property(nonatomic, retain) IBOutlet UIToolbar* toolbarView;
+@property(nonatomic, strong) IBOutlet UIView* containerView;
+@property(nonatomic, strong) IBOutlet UIToolbar* toolbarView;
 @property(nonatomic, assign, readonly) web::WebState* webState;
 
 // Initializes a new ViewController from |MainView.xib| using the given
 // |browserState|.
 - (instancetype)initWithBrowserState:(web::BrowserState*)browserState;
 
 @end
 
 #endif  // IOS_WEB_SHELL_VIEW_CONTROLLER_H_