[ObjC ARC] Converts ios/chrome/app/startup:startup to ARC.

ios/chrome/app/startup/background_upload_alert.mm

 // Copyright (c) 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.
 
 #include "ios/chrome/app/startup/background_upload_alert.h"
 
 #include "ios/chrome/browser/experimental_flags.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation BackgroundUploadAlert
 
 + (void)setupBackgroundUploadAlert {
   if (experimental_flags::IsAlertOnBackgroundUploadEnabled()) {
     if ([UIApplication instancesRespondToSelector:
                            @selector(registerUserNotificationSettings:)]) {
       [[UIApplication sharedApplication]
           registerUserNotificationSettings:
               [UIUserNotificationSettings
                   settingsForTypes:UIUserNotificationTypeAlert |
                                    UIUserNotificationTypeBadge |
                                    UIUserNotificationTypeSound
                         categories:nil]];
     }
   }
 }
 
 @end