Make sure WindowObservers are removed from window before destruction

ui/aura/window_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_AURA_WINDOW_OBSERVER_H_
 #define UI_AURA_WINDOW_OBSERVER_H_
 
 #include "base/basictypes.h"
 #include "ui/aura/aura_export.h"
 
@@ skipping 69 matching lines @@
   virtual void OnWindowStackingChanged(Window* window) {}
 
   // Invoked when a region of |window| is scheduled to be redrawn.
   virtual void OnWindowPaintScheduled(Window* window,
                                       const gfx::Rect& region) {}
 
   // Invoked when the Window is being destroyed (i.e. from the start of its
   // destructor). This is called before the window is removed from its parent.
   virtual void OnWindowDestroying(Window* window) {}
 
-  // Invoked when the Window has been destroyed (i.e. at the end of its
-  // destructor). This is called after the window is removed from its parent.
+  // Invoked when the Window has been destroyed (i.e. at the end of
+  // its destructor). This is called after the window is removed from
+  // its parent.  Window automatically removes its WindowObservers
+  // before calling this method, so the following code is no op.
+  //
+  // void MyWindowObserver::OnWindowDestroyed(aura::Window* window) {
+  //    window->RemoveObserver(this);
+  // }
   virtual void OnWindowDestroyed(Window* window) {}
 
   // Called when a Window has been added to a RootWindow.
   virtual void OnWindowAddedToRootWindow(Window* window) {}
 
   // Called when a Window is about to be removed from a RootWindow.
   virtual void OnWindowRemovingFromRootWindow(Window* window) {}
 
   // Called when a transient child is added to |window|.
   virtual void OnAddTransientChild(Window* window, Window* transient) {}
 
   // Called when a transient child is removed from |window|.
   virtual void OnRemoveTransientChild(Window* window, Window* transient) {}
 
  protected:
   virtual ~WindowObserver() {}
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_WINDOW_OBSERVER_H_