Don't let RenderScrollbarPart have padding.

Source/core/rendering/RenderScrollbarPart.h

Index: Source/core/rendering/RenderScrollbarPart.h
diff --git a/Source/core/rendering/RenderScrollbarPart.h b/Source/core/rendering/RenderScrollbarPart.h
index fa6695ab9afc8b76846ce824653e0956989561d8..73f1b6bd7f1100468be7a2071d7cc817c7a141a8 100644
--- a/Source/core/rendering/RenderScrollbarPart.h
+++ b/Source/core/rendering/RenderScrollbarPart.h
@@ -68,6 +68,15 @@ private:
 
     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();