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

ui/views/controls/scrollbar/overlay_scroll_bar.h

 // Copyright 2013 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_OVERLAY_SCROLL_BAR_H_
 #define UI_VIEWS_CONTROLS_SCROLLBAR_OVERLAY_SCROLL_BAR_H_
 
 #include "base/macros.h"
 #include "ui/views/controls/scrollbar/base_scroll_bar.h"
 #include "ui/views/controls/scrollbar/base_scroll_bar_thumb.h"
@@ skipping 18 matching lines @@
   void Layout() override;
   void OnMouseEntered(const ui::MouseEvent& event) override;
   void OnMouseExited(const ui::MouseEvent& event) override;
 
  private:
   class Thumb : public BaseScrollBarThumb {
    public:
     explicit Thumb(OverlayScrollBar* scroll_bar);
     ~Thumb() override;
 
+    void Init();
+
    protected:
     // BaseScrollBarThumb:
     gfx::Size GetPreferredSize() const override;
     void OnPaint(gfx::Canvas* canvas) override;
     void OnBoundsChanged(const gfx::Rect& previous_bounds) override;
     void OnStateChanged() override;
 
    private:
     OverlayScrollBar* scroll_bar_;
 
     DISALLOW_COPY_AND_ASSIGN(Thumb);
   };
   friend class Thumb;
 
   // Shows this (effectively, the thumb) without delay.
   void Show();
   // Hides this with a delay.
   void Hide();
   // Starts a countdown that hides this when it fires.
   void StartHideCountdown();
 
   base::Timer hide_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(OverlayScrollBar);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_SCROLLBAR_OVERLAY_SCROLL_BAR_H_