Fix crash when locking screen while adding a user to an existing session.

chrome/browser/chromeos/login/ui/login_display_host_impl.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 "chrome/browser/chromeos/login/ui/login_display_host_impl.h"
 
 #include <vector>
 
 #include "ash/audio/sounds.h"
 #include "ash/desktop_background/desktop_background_controller.h"
@@ skipping 898 matching lines @@
   if (shutting_down_)
     return;
 
   shutting_down_ = true;
   registrar_.RemoveAll();
   base::MessageLoop::current()->DeleteSoon(FROM_HERE, this);
   if (post_quit_task)
     base::MessageLoop::current()->Quit();
 
   if (!completion_callback_.is_null())
-    completion_callback_.Run();
+    base::MessageLoop::current()->PostTask(FROM_HERE, completion_callback_);

[dzhioev (left Google), 2015/09/24 23:51:17]

Can you please explain, how this helps to prevent the crash? (and at which point
of locking process ShutdownDisplayHost() is invoked?) 

 }
 
 void LoginDisplayHostImpl::ScheduleWorkspaceAnimation() {
   if (ash::Shell::GetContainer(ash::Shell::GetPrimaryRootWindow(),
                                ash::kShellWindowId_DesktopBackgroundContainer)
           ->children()
           .empty()) {
     // If there is no background window, don't perform any animation on the
     // default and background layer because there is nothing behind it.
     return;
@@ skipping 326 matching lines @@
 
   locale_util::SwitchLanguageCallback callback(
       base::Bind(&OnLanguageSwitchedCallback, base::Passed(data.Pass())));
 
   // Load locale keyboards here. Hardware layout would be automatically enabled.
   locale_util::SwitchLanguage(locale, true, true /* login_layouts_only */,
                               callback, ProfileManager::GetActiveUserProfile());
 }
 
 }  // namespace chromeos