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

third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.h

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 63 matching lines @@
                             ScrollbarPart,
                             bool painting = false) override;
   IntRect trackRect(const ScrollbarThemeClient&,
                     bool painting = false) override;
   int thumbThickness(const ScrollbarThemeClient&) override;
   int thumbThickness() { return m_thumbThickness; }
 
   void paintThumb(GraphicsContext&, const Scrollbar&, const IntRect&) override;
   ScrollbarPart hitTest(const ScrollbarThemeClient&, const IntPoint&) override;
 
+  bool usesNinePatchThumbResource() const override;
+  IntSize ninePatchThumbCanvasSize(const ScrollbarThemeClient&) const override;
+  IntRect ninePatchThumbAperture(const ScrollbarThemeClient&) const override;
+
   static ScrollbarThemeOverlay& mobileTheme();
 
  private:
   int m_thumbThickness;
   int m_scrollbarMargin;
   HitTestBehavior m_allowHitTest;
   Color m_color;
   const bool m_useSolidColor;
 };
 
 }  // namespace blink
 
 #endif