Make system modal check more strict

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_
 
 #include <vector>
 
 #include "ash/ash_export.h"
@@ skipping 32 matching lines @@
   void OnWindowAddedToLayout(aura::Window* child) override;
   void OnWillRemoveWindowFromLayout(aura::Window* child) override;
   void SetChildBounds(aura::Window* child,
                       const gfx::Rect& requested_bounds) override;
 
   // Overridden from aura::WindowObserver:
   void OnWindowPropertyChanged(aura::Window* window,
                                const void* key,
                                intptr_t old) override;
   void OnWindowDestroying(aura::Window* window) override;
+  void OnWindowVisibilityChanged(aura::Window* window, bool visible) override;
 
   // Overridden from keyboard::KeyboardControllerObserver:
   void OnKeyboardBoundsChanging(const gfx::Rect& new_bounds) override;
 
   // True if the window is either contained by the top most modal window,
   // or contained by its transient children.
   bool IsPartOfActiveModalWindow(aura::Window* window);
 
   // Activates next modal window if any. Returns false if there
   // are no more modal windows in this layout manager.
@@ skipping 39 matching lines @@
 
   // A stack of modal windows. Only the topmost can receive events.
   std::vector<aura::Window*> modal_windows_;
 
   DISALLOW_COPY_AND_ASSIGN(SystemModalContainerLayoutManager);
 };
 
 }  // namespace ash
 
 #endif  // ASH_WM_SYSTEM_MODAL_CONTAINER_LAYOUT_MANAGER_H_