Fix event targeting for overlay scrollbar thumbs (in native UI).

ui/views/controls/scrollbar/base_scroll_bar.h

Index: ui/views/controls/scrollbar/base_scroll_bar.h
diff --git a/ui/views/controls/scrollbar/base_scroll_bar.h b/ui/views/controls/scrollbar/base_scroll_bar.h
index 50b5fb64a840e2e8e781a818a4483c7383e89f4c..9355e7cd87a0e1a96496360cdb486153ba2d463a 100644
--- a/ui/views/controls/scrollbar/base_scroll_bar.h
+++ b/ui/views/controls/scrollbar/base_scroll_bar.h
@@ -32,9 +32,11 @@ class VIEWS_EXPORT BaseScrollBar : public ScrollBar,
                                    public ContextMenuController,
                                    public MenuDelegate {
  public:
-  BaseScrollBar(bool horizontal, BaseScrollBarThumb* thumb);
+  explicit BaseScrollBar(bool horizontal);
   ~BaseScrollBar() override;
 
+  void SetThumb(BaseScrollBarThumb* thumb);
+
   // Get the bounds of the "track" area that the thumb is free to slide within.
   virtual gfx::Rect GetTrackBounds() const = 0;