Use nine-patch resource for drawing Aura overlay scrollbar thumb.

cc/blink/scrollbar_impl.h

 // Copyright 2014 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 CC_BLINK_SCROLLBAR_IMPL_H_
 #define CC_BLINK_SCROLLBAR_IMPL_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 23 matching lines @@
   gfx::Point Location() const override;
   int ThumbThickness() const override;
   int ThumbLength() const override;
   gfx::Rect TrackRect() const override;
   float ThumbOpacity() const override;
   bool NeedsPaintPart(cc::ScrollbarPart part) const override;
   void PaintPart(cc::PaintCanvas* canvas,
                  cc::ScrollbarPart part,
                  const gfx::Rect& content_rect) override;
 
+  bool UsesNinePatchThumbResource() const override;
+  gfx::Size NinePatchThumbCanvasSize() const override;
+  gfx::Rect NinePatchThumbAperture() const override;
+
  private:
   std::unique_ptr<blink::WebScrollbar> scrollbar_;
   blink::WebScrollbarThemePainter painter_;
   std::unique_ptr<blink::WebScrollbarThemeGeometry> geometry_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarImpl);
 };
 
 }  // namespace cc_blink
 
 #endif  // CC_BLINK_SCROLLBAR_IMPL_H_