Refactor setup of background upload alert from MC to separate file.

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"
+
+@implementation BackgroundUploadAlert
+
++ (void)setupBackgroundUploadAlert {
+  if (experimental_flags::IsAlertOnBackgroundUploadEnabled()) {
+    if ([UIApplication instancesRespondToSelector:
+                           @selector(registerUserNotificationSettings:)]) {
+      [[UIApplication sharedApplication]
+          registerUserNotificationSettings:
+              [UIUserNotificationSettings
+                  settingsForTypes:UIUserNotificationTypeAlert |
+                                   UIUserNotificationTypeBadge |
+                                   UIUserNotificationTypeSound
+                        categories:nil]];
+    }
+  }
+}
+
+@end