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

ui/views/controls/scrollbar/scroll_bar_views.cc

Index: ui/views/controls/scrollbar/scroll_bar_views.cc
diff --git a/ui/views/controls/scrollbar/scroll_bar_views.cc b/ui/views/controls/scrollbar/scroll_bar_views.cc
index ee975749ba2f431f5f90c7fa998158eb81a85428..066a56c5ca61d20e3cd571f149657333d6e9dae0 100644
--- a/ui/views/controls/scrollbar/scroll_bar_views.cc
+++ b/ui/views/controls/scrollbar/scroll_bar_views.cc
@@ -202,7 +202,8 @@ ui::NativeTheme::State ScrollBarThumb::GetNativeThemeState() const {
 const char ScrollBarViews::kViewClassName[] = "ScrollBarViews";
 
 ScrollBarViews::ScrollBarViews(bool horizontal)
-    : BaseScrollBar(horizontal, new ScrollBarThumb(this)) {
+    : BaseScrollBar(horizontal) {
+  SetThumb(new ScrollBarThumb(this));
   if (horizontal) {
     prev_button_ = new ScrollBarButton(this, ScrollBarButton::LEFT);
     next_button_ = new ScrollBarButton(this, ScrollBarButton::RIGHT);