Fix two crash and potential leak&crash

ash/wm/system_modal_container_layout_manager.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 ASH_WM_SYSTEM_MODAL_CONTAINER_LAYOUT_MANAGER_H_
 #define ASH_WM_SYSTEM_MODAL_CONTAINER_LAYOUT_MANAGER_H_
 #pragma once
 
 #include <vector>
 
@@ skipping 36 matching lines @@
   virtual void OnWillRemoveWindowFromLayout(aura::Window* child) OVERRIDE;
   virtual void OnChildWindowVisibilityChanged(aura::Window* child,
                                               bool visibile) OVERRIDE;
   virtual void SetChildBounds(aura::Window* child,
                               const gfx::Rect& requested_bounds) OVERRIDE;
 
   // Overridden from aura::WindowObserver:
   virtual void OnWindowPropertyChanged(aura::Window* window,
                                        const void* key,
                                        intptr_t old) OVERRIDE;
+  virtual void OnWindowDestroying(aura::Window* window) OVERRIDE;
 
   // Overridden from ui::ImplicitAnimationObserver:
   virtual void OnImplicitAnimationsCompleted() OVERRIDE;
 
   // Overridden from SystemModalContainerEventFilterDelegate:
   virtual bool CanWindowReceiveEvents(aura::Window* window) OVERRIDE;
 
  private:
   void AddModalWindow(aura::Window* window);
   void RemoveModalWindow(aura::Window* window);
@@ skipping 21 matching lines @@
   // added, and removed when the last is closed.
   scoped_ptr<aura::EventFilter> modality_filter_;
 
   DISALLOW_COPY_AND_ASSIGN(SystemModalContainerLayoutManager);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_WM_SYSTEM_MODAL_CONTAINER_LAYOUT_MANAGER_H_