Don't let RenderScrollbarPart have padding.

Source/core/rendering/RenderScrollbarPart.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 50 matching lines @@
     virtual void styleDidChange(StyleDifference, const RenderStyle* oldStyle) OVERRIDE;
     virtual void imageChanged(WrappedImagePtr, const IntRect* = 0) OVERRIDE;
 
 private:
     RenderScrollbarPart(RenderScrollbar*, ScrollbarPart);
 
     virtual void computePreferredLogicalWidths() OVERRIDE;
 
     virtual bool supportsPartialLayout() const OVERRIDE { return false; }
 
+    virtual LayoutUnit paddingTop() const OVERRIDE { return LayoutUnit(); }
+    virtual LayoutUnit paddingBottom() const OVERRIDE { return LayoutUnit(); }
+    virtual LayoutUnit paddingLeft() const OVERRIDE { return LayoutUnit(); }
+    virtual LayoutUnit paddingRight() const OVERRIDE { return LayoutUnit(); }
+    virtual LayoutUnit paddingBefore() const OVERRIDE { return LayoutUnit(); }
+    virtual LayoutUnit paddingAfter() const OVERRIDE { return LayoutUnit(); }
+    virtual LayoutUnit paddingStart() const OVERRIDE { return LayoutUnit(); }
+    virtual LayoutUnit paddingEnd() const OVERRIDE { return LayoutUnit(); }

[Julien - ping for review, 2014/02/19 16:25:14]

Having a comment as to why we do that would be nice (it's because we can't
resolve percentage padding as we may not have a containing block).

Note that you could solve the problem by just returning
minimumValueForLength(rightLength, 0) which I would be more comfortable with as
it doesn't disable paddings. That would mean fixing the description.

[mstensho (USE GERRIT), 2014/02/22 20:54:50]

Done.

Simply returning 0 is fine, since scroll bar corners aren't actually laid out,
so I prefer to keep it simple (return 0), since nobody cares.

+
     void layoutHorizontalPart();
     void layoutVerticalPart();
 
     void computeScrollbarWidth();
     void computeScrollbarHeight();
 
     RenderScrollbar* m_scrollbar;
     ScrollbarPart m_part;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderScrollbarPart, isRenderScrollbarPart());
 
 } // namespace WebCore
 
 #endif // RenderScrollbarPart_h