Fix scrollbar buttons on ChromeOS.

third_party/WebKit/public/platform/WebThemeEngine.h

 /*
  * Copyright (C) 2010 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 134 matching lines @@
         SliderExtraParams slider;
         InnerSpinButtonExtraParams innerSpin;
         ProgressBarExtraParams progressBar;
     };
 
     // Gets the size of the given theme part. For variable sized items
     // like vertical scrollbar thumbs, the width will be the required width of
     // the track while the height will be the minimum height.
     virtual WebSize getSize(Part) { return WebSize(); }
 
+    // Returns whether or not buttons should be drawn on the scrollbars.
+    virtual bool hasScrollbarButtons() { return true; }
+
     struct ScrollbarStyle {
         int thumbThickness;
         int scrollbarMargin;
         WebColor color;
     };
 
     // Gets the overlay scrollbar style. Used for mobile theme.
     virtual void getOverlayScrollbarStyle(ScrollbarStyle*) { }
 
     // Paint the given the given theme part.
     virtual void paint(WebCanvas*, Part, State, const WebRect&, const ExtraParams*) { }
 };
 
 } // namespace blink
 
 #endif