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

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

 /*
  * Copyright (C) 2008 Apple 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
@@ skipping 165 matching lines @@
   virtual IntRect constrainTrackRectToTrackPieces(const ScrollbarThemeClient&,
                                                   const IntRect& rect) {
     return rect;
   }
 
   virtual void registerScrollbar(ScrollbarThemeClient&) {}
   virtual void unregisterScrollbar(ScrollbarThemeClient&) {}
 
   virtual bool isMockTheme() const { return false; }
 
+  virtual bool usesNinePatchThumbResource() const { return false; }
+
+  // For a nine-patch scrollbar, this defines the painting canvas size which the
+  // painting code will use to paint the scrollbar into. The actual scrollbar
+  // dimensions will be ignored for purposes of painting since the resource can
+  // be then resized without a repaint.
+  virtual IntSize ninePatchThumbCanvasSize(const ScrollbarThemeClient&) const {
+    NOTREACHED();
+    return IntSize();
+  }
+
+  // For a nine-patch resource, the aperture defines the size of the center
+  // patch that will be stretched out.
+  virtual IntRect ninePatchThumbAperture(const ScrollbarThemeClient&) const {
+    NOTREACHED();
+    return IntRect();
+  }
+
   static ScrollbarTheme& theme();
 
   static void setMockScrollbarsEnabled(bool flag);
   static bool mockScrollbarsEnabled();
 
  protected:
   virtual int tickmarkBorderWidth() { return 0; }
   static DisplayItem::Type buttonPartToDisplayItemType(ScrollbarPart);
   static DisplayItem::Type trackPiecePartToDisplayItemType(ScrollbarPart);
 
  private:
   static ScrollbarTheme&
   nativeTheme();  // Must be implemented to return the correct theme subclass.
   static bool gMockScrollbarsEnabled;
 };
 
 }  // namespace blink
 #endif