Fix two crash and potential leak&crash

ash/wm/system_modal_container_layout_manager.cc

 // 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.
 
 #include "ash/wm/system_modal_container_layout_manager.h"
 
 #include "ash/ash_switches.h"
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/wm/system_modal_container_event_filter.h"
@@ skipping 103 matching lines @@
   if (key != aura::client::kModalKey)
     return;
 
   if (window->GetProperty(aura::client::kModalKey) != ui::MODAL_TYPE_NONE) {
     AddModalWindow(window);
   } else if (static_cast<ui::ModalType>(old) != ui::MODAL_TYPE_NONE) {
     RemoveModalWindow(window);
   }
 }
 
+void SystemModalContainerLayoutManager::OnWindowDestroying(
+    aura::Window* window) {
+  if (modal_screen_ && modal_screen_->GetNativeView() == window)
+    modal_screen_ = NULL;
+}
+
+
 ////////////////////////////////////////////////////////////////////////////////
 // SystemModalContainerLayoutManager,
 //     ui::ImplicitAnimationObserver implementation:
 
 void SystemModalContainerLayoutManager::OnImplicitAnimationsCompleted() {
   if (modal_screen_ && !modal_screen_->GetNativeView()->layer()->ShouldDraw())
     DestroyModalScreen();
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ skipping 69 matching lines @@
 
   Shell::GetInstance()->RemoveRootWindowEventFilter(modality_filter_.get());
   ui::ScopedLayerAnimationSettings settings(
       modal_screen_->GetNativeView()->layer()->GetAnimator());
   settings.AddObserver(this);
   modal_screen_->GetNativeView()->layer()->SetOpacity(0.0f);
 }
 
 }  // namespace internal
 }  // namespace ash