Reland r256396 Add views implementation for AXTreeSource.

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 gfx {
 class Rect;
 }
 
 namespace views {
 
 class Widget;
+class View;
 
 // Observers can listen to various events on the Widgets.
 class VIEWS_EXPORT WidgetObserver {
  public:
   // The closing notification is sent immediately in response to (i.e. in the
   // same call stack as) a request to close the Widget (via Close() or
   // CloseNow()).
   virtual void OnWidgetClosing(Widget* widget) {}
 
   // Invoked after notification is received from the event loop that the native
@@ skipping 11 matching lines @@
 
   virtual void OnWidgetVisibilityChanging(Widget* widget, bool visible) {}
 
   virtual void OnWidgetVisibilityChanged(Widget* widget, bool visible) {}
 
   virtual void OnWidgetActivationChanged(Widget* widget, bool active) {}
 
   virtual void OnWidgetBoundsChanged(Widget* widget,
                                      const gfx::Rect& new_bounds) {}
 
+  // Called after a descendant view of |widget| is removed.
+  virtual void OnViewRemoved(Widget* widget, View* view) {}
+
  protected:
   virtual ~WidgetObserver() {}
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_WIDGET_OBSERVER_H_