aura: Add Ctrl-Shift-Q for quit and Ctrl-Shift-L for lock in chromeos.

ash/app_list/app_list.cc

Index: ash/app_list/app_list.cc
diff --git a/ash/app_list/app_list.cc b/ash/app_list/app_list.cc
index ed593f7b2b29e4117a9c5ca07d60763c50eb6778..587959994da78c57de54f1d6d8c4aef322f9096e 100644
--- a/ash/app_list/app_list.cc
+++ b/ash/app_list/app_list.cc
@@ -120,8 +120,11 @@ void AppList::ScheduleAnimation() {
   layer->SetBounds(GetPreferredBounds(is_visible_));
   layer->SetOpacity(is_visible_ ? 1.0 : 0.0);
 
-  ui::Layer* default_container_layer = Shell::GetInstance()->GetContainer(
-      internal::kShellWindowId_DefaultContainer)->layer();
+  aura::Window* default_container = Shell::GetInstance()->GetContainer(
+      internal::kShellWindowId_DefaultContainer);
+  if (!default_container)
+    return;
+  ui::Layer* default_container_layer = default_container->layer();
   ui::ScopedLayerAnimationSettings default_container_animation(
       default_container_layer->GetAnimator());
   default_container_layer->SetOpacity(is_visible_ ? 0.0 : 1.0);