aura: Some changes in Window and Desktop.

ui/aura/window_observer.h

+// Copyright (c) 2011 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 UI_AURA_WINDOW_OBSERVER_H_
+#define UI_AURA_WINDOW_OBSERVER_H_
+#pragma once
+
+#include "ui/aura/aura_export.h"
+
+namespace aura {
+
+class Window;
+
+class AURA_EXPORT WindowObserver {
+ public:
+  virtual void OnWindowClosing(Window* window) {}
+  virtual void OnWindowVisibilityChanged(Window* window) {}
+
+ protected:
+  virtual ~WindowObserver() {}
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_WINDOW_OBSERVER_H_