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

ios/chrome/browser/ui/show_mail_composer_util.mm

 // Copyright 2013 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/chrome/browser/ui/show_mail_composer_util.h"
 
 #include <UIKit/UIKit.h>
 
 #include "base/logging.h"
-#include "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
 #include "ios/chrome/browser/ui/commands/UIKit+ChromeExecuteCommand.h"
 #include "ios/chrome/browser/ui/commands/show_mail_composer_command.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 void ShowMailComposer(const base::string16& to_recipient,
                       const base::string16& subject,
                       const base::string16& body,
                       const base::string16& title,
                       const base::FilePath& text_file_to_attach,
                       int email_not_configured_alert_title_id,
                       int email_not_configured_alert_message_id) {
-  base::scoped_nsobject<ShowMailComposerCommand>
-  command([[ShowMailComposerCommand alloc]
+  ShowMailComposerCommand* command = [[ShowMailComposerCommand alloc]
                    initWithToRecipient:base::SysUTF16ToNSString(to_recipient)
                                subject:base::SysUTF16ToNSString(subject)
                                   body:base::SysUTF16ToNSString(body)
         emailNotConfiguredAlertTitleId:email_not_configured_alert_title_id
-      emailNotConfiguredAlertMessageId:email_not_configured_alert_message_id]);
+      emailNotConfiguredAlertMessageId:email_not_configured_alert_message_id];
   [command setTextFileToAttach:text_file_to_attach];
   UIWindow* main_window = [[UIApplication sharedApplication] keyWindow];
   DCHECK(main_window);
   [main_window.rootViewController chromeExecuteCommand:command];
 }