Fixed browser crash on trying to access the locked screen mode

On trying to access the locked screen mode in split screen (tablet mode), the browser crashes due to NULL pointer reference.

Bug: b/119853019
Change-Id: Idaf715a9d48f3979243b83bd9fa9e59706f09122
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1685101Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Aya Elsayed <ayaelattar@google.com>
Cr-Commit-Position: refs/heads/master@{#675694}

ash/wm/splitview/split_view_divider.cc

@@ -10,6 +10,7 @@
 #include "ash/public/cpp/shell_window_ids.h"
 #include "ash/screen_util.h"
 #include "ash/shell.h"
+#include "ash/wm/desks/desks_util.h"
 #include "ash/wm/splitview/split_view_constants.h"
 #include "ash/wm/splitview/split_view_controller.h"
 #include "ash/wm/splitview/split_view_divider_handler_view.h"
@@ -325,11 +326,10 @@ void SplitViewDivider::UpdateDividerBounds() {
 }
 
 gfx::Rect SplitViewDivider::GetDividerBoundsInScreen(bool is_dragging) {
-  aura::Window* root_window =
-      divider_widget_->GetNativeWindow()->GetRootWindow();
   const gfx::Rect work_area_bounds_in_screen =
       screen_util::GetDisplayWorkAreaBoundsInScreenForActiveDeskContainer(
-          root_window);
+          Shell::GetPrimaryRootWindow()->GetChildById(
+              desks_util::GetActiveDeskContainerId()));
   const int divider_position = controller_->divider_position();
   const OrientationLockType screen_orientation = GetCurrentScreenOrientation();
   return GetDividerBoundsInScreen(work_area_bounds_in_screen,