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

third_party/WebKit/Source/platform/exported/WebScrollbarThemePainter.cpp

 /*
  * 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 150 matching lines @@
 }
 
 bool WebScrollbarThemePainter::trackNeedsRepaint() const {
   return m_scrollbar->trackNeedsRepaint();
 }
 
 bool WebScrollbarThemePainter::thumbNeedsRepaint() const {
   return m_scrollbar->thumbNeedsRepaint();
 }
 
+bool WebScrollbarThemePainter::usesNinePatchThumbResource() const {
+  return m_theme->usesNinePatchThumbResource();
+}
+
 }  // namespace blink