[Merge to M115]snap-group: Fix crash on mouse hover on the divider

`kebab_button_` is not initialized to nullptr and it will cause crash on mouse hover as it may potentially contain some garbage.

Fixed: b/286212523
Test: Manual
Change-Id: I26c5aae26bb052a9dd89523dfd23b23cbb05a0ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4600055
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Michele Fan <michelefan@chromium.org>
Cr-Commit-Position: refs/branch-heads/5790@{#510}
Cr-Branched-From: 1d71a33-refs/heads/main@{#1148114}

ash/wm/splitview/split_view_divider_view.h

@@ -73,7 +73,7 @@ class SplitViewDividerView : public views::View,
 
   // A vertical 3-dot button that shows on the split view divider when
   // `ShouldAutomaticallyGroupOnWindowsSnappedInClamshell()` is true.
-  raw_ptr<IconButton, ExperimentalAsh> kebab_button_;
+  raw_ptr<IconButton, ExperimentalAsh> kebab_button_ = nullptr;
 
   // The snap group expanded menu widget and its contents view.
   views::UniqueWidgetPtr snap_group_expanded_menu_widget_;