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

ash/wm/activation_controller.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/activation_controller.h"
 
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/wm/window_modality_controller.h"
 #include "ash/wm/window_util.h"
@@ skipping 189 matching lines @@
 void ActivationController::ActivateNextWindow(aura::Window* window) {
   if (IsActiveWindow(window))
     ActivateWindow(GetTopmostWindowToActivate(window));
 }
 
 aura::Window* ActivationController::GetTopmostWindowToActivate(
     aura::Window* ignore) const {
   const aura::Window* container =
       default_container_for_test_ ? default_container_for_test_ :
           GetContainer(kShellWindowId_DefaultContainer);
+  if (!container)
+    return NULL;
   for (aura::Window::Windows::const_reverse_iterator i =
            container->children().rbegin();
        i != container->children().rend();
        ++i) {
     if (*i != ignore && CanActivateWindow(*i))
       return *i;
   }
   return NULL;
 }
 
 }  // namespace internal
 }  // namespace ash