tablet_mode: Fix crash on dragging split divider at tablet mode

This CL fixed the crash when quickly dragging the split bar at tablet
mode with multiple displays by setting the property kLockedToRootKey to
true for the native window of the split bar. By doing this, we can make
sure that the split divider will always stay at the the display where it's created.

Fixed: 1316960
Test: manual
Change-Id: Ic94c2c90a3ab69de4f894b241f6ef0dacf9d13a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3716823
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Connie Xu <conniekxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1017822}

ash/wm/splitview/split_view_divider.cc

@@ -18,6 +18,7 @@
 #include "ash/wm/splitview/split_view_divider_handler_view.h"
 #include "ash/wm/splitview/split_view_utils.h"
 #include "ash/wm/tablet_mode/tablet_mode_controller.h"
+#include "ash/wm/window_properties.h"
 #include "ash/wm/window_util.h"
 #include "base/containers/contains.h"
 #include "base/task/sequenced_task_runner.h"
@@ -494,6 +495,7 @@ void SplitViewDivider::CreateDividerWidget(SplitViewController* controller) {
   divider_view_ = divider_widget_->SetContentsView(
       std::make_unique<DividerView>(controller, this));
   divider_widget_->SetBounds(GetDividerBoundsInScreen(false /* is_dragging */));
+  divider_widget_->GetNativeWindow()->SetProperty(kLockedToRootKey, true);
   divider_widget_->Show();
 }