Convert ios_web_shell to ARC.

ios/web/shell/shell_web_client.mm

 // 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.
 
 #include "ios/web/shell/shell_web_client.h"
 
 #import <UIKit/UIKit.h>
 
 #include "ios/web/public/user_agent.h"
 #include "ios/web/shell/shell_web_main_parts.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 ShellWebClient::ShellWebClient() {
 }
 
 ShellWebClient::~ShellWebClient() {
 }
 
 WebMainParts* ShellWebClient::CreateWebMainParts() {
   web_main_parts_.reset(new ShellWebMainParts);
@@ skipping 38 matching lines @@
                                               block_callback.Run(true);
                                             }]];
   }
   [[UIApplication sharedApplication].keyWindow.rootViewController
       presentViewController:alert
                    animated:YES
                  completion:nil];
 }
 
 }  // namespace web