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

ash/wm/stacking_controller.cc

Index: ash/wm/stacking_controller.cc
diff --git a/ash/wm/stacking_controller.cc b/ash/wm/stacking_controller.cc
index 3802440be6da86583f1893ddfd66be6879552de9..3590da7c016353da606f57d86438d7c860471bca 100644
--- a/ash/wm/stacking_controller.cc
+++ b/ash/wm/stacking_controller.cc
@@ -128,13 +128,15 @@ aura::Window* StackingController::GetSystemModalContainer(
 
   // 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();
+
+  // Transient parent for window can be NULL for alerts from background pages
+  // assume that all such alerts belong to user session.
+  int window_container_id = window->transient_parent() ?

[sky, 2013/05/30 14:53:47]

This code would be less mysterious if you made it early out, eg:

if (!window->transient_parent())
  return GetContainerById(root, internal::kShellWindowId_SystemModalContainer);
}

In fact you could combine this with the if on 122 (keeping your comment).

[Dmitry Polukhin, 2013/05/30 15:09:02]

Done.

+      window->transient_parent()->parent()->id() :
+      internal::kShellWindowId_WorkspaceContainer;
 
   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 {