Extending the NthIndexCache to support caching for the type of index.

Source/core/css/SiblingTraversalStrategies.h

Index: Source/core/css/SiblingTraversalStrategies.h
diff --git a/Source/core/css/SiblingTraversalStrategies.h b/Source/core/css/SiblingTraversalStrategies.h
index 1e008dba5fda36e1c053fe4b5b43ba61e10e51af..9a2e2fdfa11c7bd0751338f3e24f496d7b76b72f 100644
--- a/Source/core/css/SiblingTraversalStrategies.h
+++ b/Source/core/css/SiblingTraversalStrategies.h
@@ -93,6 +93,8 @@ inline int DOMSiblingTraversalStrategy::countElementsBefore(Element& element) co
 
 inline int DOMSiblingTraversalStrategy::countElementsOfTypeBefore(Element& element, const QualifiedName& type) const
 {
+    if (NthIndexCache* nthIndexCache = element.document().nthIndexCache())
+        return nthIndexCache->nthChildIndexOfType(element, type) - 1;
     int count = 0;
     for (const Element* sibling = ElementTraversal::previousSibling(element, HasTagName(type)); sibling; sibling = ElementTraversal::previousSibling(*sibling, HasTagName(type)))
         ++count;
@@ -112,6 +114,9 @@ inline int DOMSiblingTraversalStrategy::countElementsAfter(Element& element) con
 
 inline int DOMSiblingTraversalStrategy::countElementsOfTypeAfter(Element& element, const QualifiedName& type) const
 {
+    if (NthIndexCache* nthIndexCache = element.document().nthIndexCache())
+        return nthIndexCache->nthLastChildIndexOfType(element, type) - 1;
+
     int count = 0;
     for (const Element* sibling = ElementTraversal::nextSibling(element, HasTagName(type)); sibling; sibling = ElementTraversal::nextSibling(*sibling, HasTagName(type)))
         ++count;