Fix two crash and potential leak&crash

ash/wm/system_modal_container_layout_manager.cc

Index: ash/wm/system_modal_container_layout_manager.cc
diff --git a/ash/wm/system_modal_container_layout_manager.cc b/ash/wm/system_modal_container_layout_manager.cc
index b96fb04e15651210eddb460536bd94c7ca395fa8..daf3c7ee4f429eb91363d665fea56688f44007f0 100644
--- a/ash/wm/system_modal_container_layout_manager.cc
+++ b/ash/wm/system_modal_container_layout_manager.cc
@@ -121,6 +121,13 @@ void SystemModalContainerLayoutManager::OnWindowPropertyChanged(
   }
 }
 
+void SystemModalContainerLayoutManager::OnWindowDestroying(
+    aura::Window* window) {
+  if (modal_screen_ && modal_screen_->GetNativeView() == window)
+    modal_screen_ = NULL;
+}
+
+
 ////////////////////////////////////////////////////////////////////////////////
 // SystemModalContainerLayoutManager,
 //     ui::ImplicitAnimationObserver implementation: