chrome: Cleanup class/struct forward declarations

chrome/browser/notifications/notifier_source.h

 // 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.
 
 #ifndef CHROME_BROWSER_NOTIFICATIONS_NOTIFIER_SOURCE_H_
 #define CHROME_BROWSER_NOTIFICATIONS_NOTIFIER_SOURCE_H_
 
 #include <memory>
 #include <vector>
 
 #include "base/macros.h"
 #include "ui/message_center/notifier_settings.h"
 
 class Profile;
 
 namespace message_center {
 struct Notifier;
-class NotifierSettingsObserver;
 }
 
 class NotifierSource {
  public:
   class Observer {
    public:
     virtual void OnIconImageUpdated(const message_center::NotifierId& id,
                                     const gfx::Image& image) = 0;
     virtual void OnNotifierEnabledChanged(const message_center::NotifierId& id,
                                           bool enabled) = 0;
@@ skipping 20 matching lines @@
   virtual void OnNotifierSettingsClosing() {}
 
   // Notifier type provided by the source.
   virtual message_center::NotifierId::NotifierType GetNotifierType() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(NotifierSource);
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_NOTIFIER_SOURCE_H_