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..93401c7a68a949c40195994f3132cdfaae96c7a2 100755
--- a/Source/core/rendering/style/StyleRareNonInheritedData.h
+++ b/Source/core/rendering/style/StyleRareNonInheritedData.h
@@ -197,6 +197,10 @@ public:
     unsigned m_justifySelf : 4; // ItemPosition
     unsigned m_justifySelfOverflowAlignment : 2; // OverflowAlignment
 
+    // ScrollBehavior. 'scroll-behavior' has 2 accepted values, but ScrollBehavior has a third
+    // value (that can only be specified using CSSOM scroll APIs) so 2 bits are needed.
+    unsigned m_scrollBehavior: 2;
+
 private:
     StyleRareNonInheritedData();
     StyleRareNonInheritedData(const StyleRareNonInheritedData&);