Cache element indices for :nth-child and :nth-last-child selectors.

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 f648cb9ef84f47a17bd54a0287e3f3988ce6dd16..e532bcfaaae28f9e89274806f9503e677e2788fb 100644
--- a/Source/core/css/SiblingTraversalStrategies.h
+++ b/Source/core/css/SiblingTraversalStrategies.h
@@ -29,8 +29,10 @@
 #ifndef SiblingTraversalStrategies_h
 #define SiblingTraversalStrategies_h
 
+#include "core/dom/Document.h"
 #include "core/dom/Element.h"
 #include "core/dom/ElementTraversal.h"
+#include "core/dom/NthIndexCache.h"
 #include "core/layout/style/LayoutStyle.h"
 
 namespace blink {
@@ -79,6 +81,9 @@ inline bool DOMSiblingTraversalStrategy::isLastOfType(Element& element, const Qu
 
 inline int DOMSiblingTraversalStrategy::countElementsBefore(Element& element) const
 {
+    if (element.document().nthIndexCache())
+        return element.document().nthIndexCache()->nthChildIndex(element) - 1;
+
     int count = 0;
     for (const Element* sibling = ElementTraversal::previousSibling(element); sibling; sibling = ElementTraversal::previousSibling(*sibling))
         count++;
@@ -96,6 +101,9 @@ inline int DOMSiblingTraversalStrategy::countElementsOfTypeBefore(Element& eleme
 
 inline int DOMSiblingTraversalStrategy::countElementsAfter(Element& element) const
 {
+    if (element.document().nthIndexCache())
+        return element.document().nthIndexCache()->nthLastChildIndex(element) - 1;
+
     int count = 0;
     for (const Element* sibling = ElementTraversal::nextSibling(element); sibling; sibling = ElementTraversal::nextSibling(*sibling))
         ++count;