Make system modal check more strict

ash/wm/system_modal_container_layout_manager.h

Index: ash/wm/system_modal_container_layout_manager.h
diff --git a/ash/wm/system_modal_container_layout_manager.h b/ash/wm/system_modal_container_layout_manager.h
index 81e2fe1de121daf3118355b8130d1efda4a6ac65..9f20e24249333e40aa8fd6777986e8f3fd70ab54 100644
--- a/ash/wm/system_modal_container_layout_manager.h
+++ b/ash/wm/system_modal_container_layout_manager.h
@@ -50,6 +50,7 @@ class ASH_EXPORT SystemModalContainerLayoutManager
                                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;