Factor out common BorderImageLength code in CSSToStyleMap

Source/core/rendering/style/NinePieceImage.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 96 matching lines @@
     void copyRepeatFrom(const NinePieceImage& other)
     {
         m_data.access()->horizontalRule = other.m_data->horizontalRule;
         m_data.access()->verticalRule = other.m_data->verticalRule;
     }
 
     void setMaskDefaults()
     {
         m_data.access()->imageSlices = LengthBox(0);
         m_data.access()->fill = true;
-        m_data.access()->borderSlices = BorderImageLengthBox();
+        m_data.access()->borderSlices = BorderImageLengthBox(Length(Auto));
     }
 
     static LayoutUnit computeOutset(const BorderImageLength& outsetSide, LayoutUnit borderSide)
     {
         if (outsetSide.isNumber())
             return outsetSide.number() * borderSide;
         return outsetSide.length().value();
     }
 
 private:
     DataRef<NinePieceImageData> m_data;
 };
 
 } // namespace WebCore
 
 #endif // NinePieceImage_h