Remove NOTIFICATION_SYSTEM_SETTING_CHANGED, switch CrosSettings to base::CallbackRegistry.

chrome/browser/chrome_notification_types.h

 // Copyright 2012 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 CHROME_BROWSER_CHROME_NOTIFICATION_TYPES_H_
 #define CHROME_BROWSER_CHROME_NOTIFICATION_TYPES_H_
 
 #include "build/build_config.h"
 #include "content/public/browser/notification_types.h"
 
@@ skipping 863 matching lines @@
   // is being deleted, so the receiver shouldn't use the screen locker
   // object.
   NOTIFICATION_SCREEN_LOCK_STATE_CHANGED,
 
   // Sent by DeviceSettingsService to indicate that the ownership status
   // changed. If you can, please use DeviceSettingsService::Observer instead.
   // Other singleton-based services can't use that because Observer
   // unregistration is impossible due to unpredictable deletion order.
   NOTIFICATION_OWNERSHIP_STATUS_CHANGED,
 
-  // This is sent to a ChromeOS settings observer when a system setting is
-  // changed. The source is the CrosSettings and the details a std::string of
-  // the changed setting.
-  NOTIFICATION_SYSTEM_SETTING_CHANGED,
-
   // Sent by SIM unlock dialog when it has finished with the process of
   // updating RequirePin setting. RequirePin setting might have been changed
   // to a new value or update might have been canceled.
   // In either case notification is sent and details contain a bool
   // that represents current value.
   NOTIFICATION_REQUIRE_PIN_SETTING_CHANGE_ENDED,
 
   // Sent by SIM unlock dialog when it has finished the EnterPin or
   // EnterPuk dialog, either because the user cancelled, or entered a
   // PIN or PUK.
@@ skipping 164 matching lines @@
   // Note:-
   // Currently only Content and Chrome define and use notifications.
   // Custom notifications not belonging to Content and Chrome should start
   // from here.
   NOTIFICATION_CHROME_END,
 };
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_CHROME_NOTIFICATION_TYPES_H_