Add new flag for Notification Center features that are not ready yet.

ui/message_center/message_center_switches.h

 // Copyright (c) 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 UI_MESSAGE_CENTER_MESSAGE_CENTER_SWITCHES_H_
 #define UI_MESSAGE_CENTER_MESSAGE_CENTER_SWITCHES_H_
 
 #include "ui/message_center/message_center_export.h"
 
 namespace message_center {
 namespace switches {
 
 // Enables rich templated notifications and NotificationCenter. In ChromeOS,
 // this flag also means the new design of message center bubble and popups.
 // TODO(mukai): Remove this flag when we don't need to provide both of designs
 // anymore (i.e. the new design becomes default and no one complains about it).
 // Note that some classes should be removed and renamed as the result of
 // removing this class.
 // Affected class list:
 //  - WebNotificationButtonView2: remove '2' suffix and replace the old one.
 //  - WebNotificationButtonViewBase: merge into WebNotificationButtonView.
 MESSAGE_CENTER_EXPORT extern const char kEnableRichNotifications[];
 MESSAGE_CENTER_EXPORT extern const char kDisableRichNotifications[];
 
+// Enables experimental features that we don't want to ship yet.
+MESSAGE_CENTER_EXPORT extern const char kEnableExperimentalNotificationUI[];
+
 }  // namespace switches
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_MESSAGE_CENTER_CONSTANTS_H_