Implement 'scroll-behavior' parsing

Source/core/rendering/style/StyleRareNonInheritedData.h

Index: Source/core/rendering/style/StyleRareNonInheritedData.h
diff --git a/Source/core/rendering/style/StyleRareNonInheritedData.h b/Source/core/rendering/style/StyleRareNonInheritedData.h
index bf09ba8fe452cb857fbd868ccf1cec48ebb2c167..33447ae866dda34a6df5ed1f3801c4d053416874 100755
--- a/Source/core/rendering/style/StyleRareNonInheritedData.h
+++ b/Source/core/rendering/style/StyleRareNonInheritedData.h
@@ -197,6 +197,8 @@ public:
     unsigned m_justifySelf : 4; // ItemPosition
     unsigned m_justifySelfOverflowAlignment : 2; // OverflowAlignment
 
+    unsigned m_scrollBehavior: 2; // ScrollBehavior

[Julien - ping for review, 2014/01/22 16:24:35]

It's bad that we are using an extra bit for no good reason: the accepted values
are "instant" or "smooth" which is really a binary choice. I don't think we
should modify the code though but maybe worth mentioning it in a comment?

[ajuma, 2014/01/22 19:50:20]

Added a comment.

+
 private:
     StyleRareNonInheritedData();
     StyleRareNonInheritedData(const StyleRareNonInheritedData&);