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

ui/views/controls/scrollbar/scroll_bar_views.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/controls/scrollbar/scroll_bar_views.h"
 
 #include "base/logging.h"
 #include "ui/events/keycodes/keyboard_codes.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/path.h"
@@ skipping 184 matching lines @@
 }
 
 }  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
 // ScrollBarViews, public:
 
 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);
 
     part_ = ui::NativeTheme::kScrollbarHorizontalTrack;
   } else {
     prev_button_ = new ScrollBarButton(this, ScrollBarButton::UP);
     next_button_ = new ScrollBarButton(this, ScrollBarButton::DOWN);
 
     part_ = ui::NativeTheme::kScrollbarVerticalTrack;
@@ skipping 118 matching lines @@
   ui::NativeTheme::ExtraParams thumb_params;
   thumb_params.scrollbar_thumb.is_hovering = false;
   gfx::Size track_size =
       theme->GetPartSize(ui::NativeTheme::kScrollbarHorizontalThumb,
                          ui::NativeTheme::kNormal, thumb_params);
 
   return std::max(track_size.height(), button_size.height());
 }
 
 }  // namespace views