chromeos: Move files in //ash/common to //ash, part 1

ash/common/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_COMMON_SYSTEM_ACCESSIBILITY_OBSERVER_H_
-#define ASH_COMMON_SYSTEM_ACCESSIBILITY_OBSERVER_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_COMMON_SYSTEM_ACCESSIBILITY_OBSERVER_H_