chromeos: Move files in //ash/common to //ash, part 2

ash/system/accessibility_observer.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_SYSTEM_ACCESSIBILITY_OBSERVER_H_
 #define ASH_SYSTEM_ACCESSIBILITY_OBSERVER_H_
 
+#include "ash/accessibility_types.h"
 #include "ash/ash_export.h"
-#include "ash/common/accessibility_types.h"
 
 namespace ash {
 
 class ASH_EXPORT AccessibilityObserver {
  public:
   virtual ~AccessibilityObserver() {}
 
   // Notifies when accessibility mode changes.
   virtual void OnAccessibilityModeChanged(
       AccessibilityNotificationVisibility notify) = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_ACCESSIBILITY_OBSERVER_H_