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

third_party/WebKit/public/platform/WebScrollbarThemeGeometry.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
  *     notice, this list of conditions and the following disclaimer in the
  *     documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE AND ITS CONTRIBUTORS "AS IS" AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL APPLE OR ITS CONTRIBUTORS BE LIABLE FOR ANY
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 #ifndef WebScrollbarThemeGeometry_h
 #define WebScrollbarThemeGeometry_h
 
 #include "WebRect.h"
+#include "WebSize.h"
 
 namespace blink {
 
 class WebScrollbar;
 
 class BLINK_PLATFORM_EXPORT WebScrollbarThemeGeometry {
  public:
   virtual ~WebScrollbarThemeGeometry() {}
 
   virtual bool hasButtons(WebScrollbar*) = 0;
   virtual bool hasThumb(WebScrollbar*) = 0;
   virtual WebRect trackRect(WebScrollbar*) = 0;
   virtual WebRect thumbRect(WebScrollbar*) = 0;
   virtual WebRect backButtonStartRect(WebScrollbar*) = 0;
   virtual WebRect backButtonEndRect(WebScrollbar*) = 0;
   virtual WebRect forwardButtonStartRect(WebScrollbar*) = 0;
   virtual WebRect forwardButtonEndRect(WebScrollbar*) = 0;
+  virtual WebSize ninePatchThumbCanvasSize(WebScrollbar*) = 0;
+  virtual WebRect ninePatchThumbAperture(WebScrollbar*) = 0;
 };
 
 }  // namespace blink
 
 #endif