Converts overview to use common ash types

ash/wm/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_WM_COMMON_WM_ACTIVATION_OBSERVER_H_
 #define ASH_WM_COMMON_WM_ACTIVATION_OBSERVER_H_
 
 #include "ash/wm/common/ash_wm_common_export.h"
 
 namespace ash {
 namespace wm {
 
 class WmWindow;
 
 // Used to track changes in activation.
 class ASH_WM_COMMON_EXPORT WmActivationObserver {
  public:
   virtual void OnWindowActivated(WmWindow* gained_active,
-                                 WmWindow* lost_active) = 0;
+                                 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.

[James Cook, 2016/05/27 00:02:51]

nice comment

[sky, 2016/05/27 03:18:03]

This is just a copy of that from ActivationObserver.

+  virtual void OnAttemptToReactivateWindow(WmWindow* request_active,
+                                           WmWindow* actual_active) {}
 
  protected:
   virtual ~WmActivationObserver() {}
 };
 
 }  // namespace wm
 }  // namespace ash
 
 #endif  // ASH_WM_COMMON_WM_ACTIVATION_OBSERVER_H_