Revert of Remove header dependencies from sequence_checker.h to sequenced_worker_pool.h

ash/common/system/tray/system_tray_notifier.h

 // Copyright 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 ASH_COMMON_SYSTEM_TRAY_SYSTEM_TRAY_NOTIFIER_H_
 #define ASH_COMMON_SYSTEM_TRAY_SYSTEM_TRAY_NOTIFIER_H_
 
 #include <string>
 
 #include "ash/ash_export.h"
 #include "ash/common/accessibility_types.h"
-#include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "base/strings/string16.h"
 #include "base/time/time.h"
 
 namespace ash {
 
 class AccessibilityObserver;
 class BluetoothObserver;
 class ClockObserver;
@@ skipping 128 matching lines @@
   base::ObserverList<TracingObserver> tracing_observers_;
   base::ObserverList<UserObserver> user_observers_;
   base::ObserverList<VirtualKeyboardObserver> virtual_keyboard_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(SystemTrayNotifier);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_TRAY_SYSTEM_TRAY_NOTIFIER_H_