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

ui/views/controls/scrollbar/cocoa_scroll_bar.mm

Index: ui/views/controls/scrollbar/cocoa_scroll_bar.mm
diff --git a/ui/views/controls/scrollbar/cocoa_scroll_bar.mm b/ui/views/controls/scrollbar/cocoa_scroll_bar.mm
index 432c43aba43d7ab14ae05aae33629532df44a144..40390cd1b95227b67b4369d2cb1166805f861ab6 100644
--- a/ui/views/controls/scrollbar/cocoa_scroll_bar.mm
+++ b/ui/views/controls/scrollbar/cocoa_scroll_bar.mm
@@ -161,7 +161,7 @@ void CocoaScrollBarThumb::OnMouseExited(const ui::MouseEvent& event) {
 // CocoaScrollBar class
 
 CocoaScrollBar::CocoaScrollBar(bool horizontal)
-    : BaseScrollBar(horizontal, new CocoaScrollBarThumb(this)),
+    : BaseScrollBar(horizontal),
       hide_scrollbar_timer_(
           FROM_HERE,
           base::TimeDelta::FromMilliseconds(kScrollbarHideTimeoutMs),
@@ -171,6 +171,7 @@ CocoaScrollBar::CocoaScrollBar(bool horizontal)
       last_contents_scroll_offset_(0),
       is_expanded_(false),
       did_start_dragging_(false) {
+  SetThumb(new CocoaScrollBarThumb(this));
   bridge_.reset([[ViewsScrollbarBridge alloc] initWithDelegate:this]);
   scroller_style_ = [ViewsScrollbarBridge getPreferredScrollerStyle];