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

ui/views/controls/scrollbar/base_scroll_bar.h

 // 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.
 
 #ifndef UI_VIEWS_CONTROLS_SCROLLBAR_BASE_SCROLL_BAR_H_
 #define UI_VIEWS_CONTROLS_SCROLLBAR_BASE_SCROLL_BAR_H_
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "ui/views/animation/scroll_animator.h"
@@ skipping 14 matching lines @@
 ///////////////////////////////////////////////////////////////////////////////
 //
 // BaseScrollBar
 //
 ///////////////////////////////////////////////////////////////////////////////
 class VIEWS_EXPORT BaseScrollBar : public ScrollBar,
                                    public ScrollDelegate,
                                    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;
 
   // An enumeration of different amounts of incremental scroll, representing
   // events sent from different parts of the UI/keyboard.
   enum ScrollAmount {
     SCROLL_NONE = 0,
     SCROLL_START,
     SCROLL_END,
     SCROLL_PREV_LINE,
@@ skipping 123 matching lines @@
   // TODO(tdresser): This should be removed when raw pixel scrolling for views
   // is enabled. See crbug.com/329354.
   gfx::Vector2dF roundoff_error_;
 
   DISALLOW_COPY_AND_ASSIGN(BaseScrollBar);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_SCROLLBAR_BASE_SCROLL_BAR_H_