Make sure WindowObservers are removed from window before destruction

ash/wm/window_state.h

 // Copyright 2013 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_WINDOW_STATE_H_
 #define ASH_WM_WINDOW_STATE_H_
 
 #include "ash/ash_export.h"
 #include "ash/wm/wm_types.h"
 #include "base/basictypes.h"
@@ skipping 233 matching lines @@
     return window_resizer_;
   }
   void set_window_resizer_(WindowResizer* window_resizer) {
     window_resizer_ = window_resizer;
   }
 
   // aura::WindowObserver overrides:
   virtual void OnWindowPropertyChanged(aura::Window* window,
                                        const void* key,
                                        intptr_t old) OVERRIDE;
-  virtual void OnWindowDestroying(aura::Window* window) OVERRIDE;
 
  private:
   // Snaps the window to left or right of the desktop with given bounds.
   void SnapWindow(WindowShowType left_or_right,
                   const gfx::Rect& bounds);
 
   // The owner of this window settings.
   aura::Window* window_;
   scoped_ptr<WindowStateDelegate> delegate_;
 
@@ skipping 33 matching lines @@
 ASH_EXPORT WindowState* GetWindowState(aura::Window* window);
 
 // const version of GetWindowState.
 ASH_EXPORT const WindowState*
 GetWindowState(const aura::Window* window);
 
 }  // namespace wm
 }  // namespace ash
 
 #endif  // ASH_WM_WINDOW_STATE_H_