Makes WindowTree::SetFocus(null) work

components/mus/ws/default_access_policy.cc

 // Copyright 2014 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 "components/mus/ws/default_access_policy.h"
 
 #include "components/mus/ws/access_policy_delegate.h"
 #include "components/mus/ws/server_window.h"
 
 namespace mus {
@@ skipping 111 matching lines @@
   return WasCreatedByThisConnection(window) ||
          delegate_->HasRootForAccessPolicy(window);
 }
 
 bool DefaultAccessPolicy::CanSetCapture(const ServerWindow* window) const {
   return WasCreatedByThisConnection(window) ||
          delegate_->HasRootForAccessPolicy(window);
 }
 
 bool DefaultAccessPolicy::CanSetFocus(const ServerWindow* window) const {
-  return WasCreatedByThisConnection(window) ||
+  return !window || WasCreatedByThisConnection(window) ||
          delegate_->HasRootForAccessPolicy(window);
 }
 
 bool DefaultAccessPolicy::CanSetClientArea(const ServerWindow* window) const {
   return WasCreatedByThisConnection(window) ||
          delegate_->HasRootForAccessPolicy(window);
 }
 
 bool DefaultAccessPolicy::CanSetCursorProperties(
     const ServerWindow* window) const {
@@ skipping 40 matching lines @@
 bool DefaultAccessPolicy::IsValidIdForNewWindow(
     const ClientWindowId& id) const {
   // Clients using DefaultAccessPolicy only see windows they have created (for
   // the embed point they choose the id), so it's ok for clients to use whatever
   // id they want.
   return true;
 }
 
 }  // namespace ws
 }  // namespace mus