Fix crash when JS alert from background page appears during lock screen

ash/wm/stacking_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/stacking_controller.h"
 
 #include "ash/display/display_controller.h"
 #include "ash/session_state_delegate.h"
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
@@ skipping 101 matching lines @@
 ////////////////////////////////////////////////////////////////////////////////
 // StackingController, private:
 
 aura::Window* StackingController::GetSystemModalContainer(
     aura::RootWindow* root,
     aura::Window* window) const {
   DCHECK(IsSystemModal(window));
 
   // If screen lock is not active and user session is active,
   // all modal windows are placed into the normal modal container.
-  if (!Shell::GetInstance()->session_state_delegate()->IsScreenLocked() &&
-      Shell::GetInstance()->session_state_delegate()->
-          IsActiveUserSessionStarted()) {
+  // In case of missing transient parent (it could happen for alerts from
+  // background pages) assume that the window belongs to user session.
+  SessionStateDelegate* session_state_delegate =
+      Shell::GetInstance()->session_state_delegate();
+  if ((!session_state_delegate->IsScreenLocked() &&
+       session_state_delegate->IsActiveUserSessionStarted()) ||
+      !window->transient_parent()) {
     return GetContainerById(root,
                             internal::kShellWindowId_SystemModalContainer);
   }
 
   // Otherwise those that originate from LockScreen container and above are
   // placed in the screen lock modal container.
-  aura::Window* lock_container =
-      GetContainerById(root, internal::kShellWindowId_LockScreenContainer);
-  int lock_container_id = lock_container->id();
   int window_container_id = window->transient_parent()->parent()->id();
-
   aura::Window* container = NULL;
-  if (window_container_id < lock_container_id) {
+  if (window_container_id < internal::kShellWindowId_LockScreenContainer) {
     container = GetContainerById(
         root, internal::kShellWindowId_SystemModalContainer);
   } else {
     container = GetContainerById(
         root, internal::kShellWindowId_LockSystemModalContainer);
   }
 
   return container;
 }
 
 // TODO(oshima): Remove this once extended desktop is on by default.
 internal::AlwaysOnTopController*
 StackingController::GetAlwaysOnTopController(aura::RootWindow* root_window) {
   internal::AlwaysOnTopController* controller =
       root_window->GetProperty(internal::kAlwaysOnTopControllerKey);
   if (!controller) {
     controller = new internal::AlwaysOnTopController;
     controller->SetAlwaysOnTopContainer(
         root_window->GetChildById(
             internal::kShellWindowId_AlwaysOnTopContainer));
     // RootWindow owns the AlwaysOnTopController object.
     root_window->SetProperty(internal::kAlwaysOnTopControllerKey, controller);
   }
   return controller;
 }
 
 }  // namespace ash