Hook up the FocusController behind a flag.

ash/wm/ash_focus_rules.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/ash_focus_rules.h"
 
 #include "ui/aura/window.h"
 
 namespace ash {
 namespace wm {
 
 ////////////////////////////////////////////////////////////////////////////////
 // AshFocusRules, public:
 
 AshFocusRules::AshFocusRules() {
 }
 
 AshFocusRules::~AshFocusRules() {
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // AshFocusRules, views::corewm::FocusRules:
 
 bool AshFocusRules::CanActivateWindow(aura::Window* window) {
-  return !!window->parent();
+  return window && !!window->parent();
 }
 
 bool AshFocusRules::CanFocusWindow(aura::Window* window) {
   aura::Window* activatable = GetActivatableWindow(window);
   return activatable->Contains(window) && window->CanFocus();
 }
 
 aura::Window* AshFocusRules::GetActivatableWindow(aura::Window* window) {
   return window;
 }
 
 aura::Window* AshFocusRules::GetFocusableWindow(aura::Window* window) {
   return window;
 }
 
 aura::Window* AshFocusRules::GetNextActivatableWindow(aura::Window* ignore) {
   return NULL;
 }
 
 aura::Window* AshFocusRules::GetNextFocusableWindow(aura::Window* ignore) {
   return GetFocusableWindow(ignore->parent());
 }
 
 }  // namespace wm
 }  // namespace ash