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

third_party/WebKit/public/platform/WebCompositorSupport.h

 /*
  * Copyright (C) 2012 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 52 matching lines @@
   virtual std::unique_ptr<WebExternalTextureLayer> createExternalTextureLayer(
       cc::TextureLayerClient*) = 0;
 
   virtual std::unique_ptr<WebImageLayer> createImageLayer() = 0;
 
   virtual std::unique_ptr<WebScrollbarLayer> createScrollbarLayer(
       std::unique_ptr<WebScrollbar>,
       WebScrollbarThemePainter,
       std::unique_ptr<WebScrollbarThemeGeometry>) = 0;
 
+  virtual std::unique_ptr<WebScrollbarLayer> createOverlayScrollbarLayer(
+      std::unique_ptr<WebScrollbar>,
+      WebScrollbarThemePainter,
+      std::unique_ptr<WebScrollbarThemeGeometry>) = 0;
+
   virtual std::unique_ptr<WebScrollbarLayer> createSolidColorScrollbarLayer(
       WebScrollbar::Orientation,
       int thumbThickness,
       int trackStart,
       bool isLeftSideVerticalScrollbar) = 0;
 
  protected:
   virtual ~WebCompositorSupport() {}
 };
 }
 
 #endif  // WebCompositorSupport_h