Converts WmActivationObserver to aura::client::ActivationChangeObserver

ash/common/wm_activation_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_WM_ACTIVATION_OBSERVER_H_
-#define ASH_COMMON_WM_ACTIVATION_OBSERVER_H_
-
-#include "ash/ash_export.h"
-
-namespace ash {
-
-class WmWindow;
-
-// Used to track changes in activation.
-class ASH_EXPORT WmActivationObserver {
- public:
-  virtual void OnWindowActivated(WmWindow* gained_active,
-                                 WmWindow* lost_active) {}
-
-  // Called when during window activation the currently active window is
-  // selected for activation. This can happen when a window requested for
-  // activation cannot be activated because a system modal window is active.
-  virtual void OnAttemptToReactivateWindow(WmWindow* request_active,
-                                           WmWindow* actual_active) {}
-
- protected:
-  virtual ~WmActivationObserver() {}
-};
-
-}  // namespace ash
-
-#endif  // ASH_COMMON_WM_ACTIVATION_OBSERVER_H_