views: Extract Widget observer into its own header file.

ui/views/widget/widget_observer.h

+// Copyright (c) 2012 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_VIEWS_WIDGET_WIDGET_OBSERVER_H_
+#define UI_VIEWS_WIDGET_WIDGET_OBSERVER_H_
+
+#include "ui/views/views_export.h"
+
+namespace views {
+
+class Widget;
+
+// Observers can listen to various events on the Widgets.
+class VIEWS_EXPORT WidgetObserver {
+ public:
+  virtual void OnWidgetClosing(Widget* widget) {}
+
+  virtual void OnWidgetVisibilityChanged(Widget* widget, bool visible) {}
+
+  virtual void OnWidgetActivationChanged(Widget* widget, bool active) {}
+
+  virtual void OnWidgetMoved(Widget* widget) {}
+
+ protected:
+  virtual ~WidgetObserver() {}
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_WIDGET_WIDGET_OBSERVER_H_