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

Source/core/dom/ContainerNode.cpp

Index: Source/core/dom/ContainerNode.cpp
diff --git a/Source/core/dom/ContainerNode.cpp b/Source/core/dom/ContainerNode.cpp
index 5ca16c88a4f7091f55fbf2ad4120deaef2db9de2..44c7705756ddbc732b48d4c6f7830b78fc6e9e93 100644
--- a/Source/core/dom/ContainerNode.cpp
+++ b/Source/core/dom/ContainerNode.cpp
@@ -34,6 +34,7 @@
 #include "core/dom/NodeLayoutStyle.h"
 #include "core/dom/NodeRareData.h"
 #include "core/dom/NodeTraversal.h"
+#include "core/dom/NthIndexCache.h"
 #include "core/dom/SelectorQuery.h"
 #include "core/dom/StaticNodeList.h"
 #include "core/dom/StyleEngine.h"
@@ -855,6 +856,7 @@ void ContainerNode::childrenChanged(const ChildrenChange& change)
         setChildNeedsStyleRecalc();
         markAncestorsWithChildNeedsStyleRecalc();
     }
+    ASSERT(!document().nthIndexCache());
 }
 
 void ContainerNode::cloneChildNodes(ContainerNode *clone)
@@ -1158,6 +1160,8 @@ PassRefPtrWillBeRawPtr<Element> ContainerNode::querySelector(const AtomicString&
     SelectorQuery* selectorQuery = document().selectorQueryCache().add(selectors, document(), exceptionState);
     if (!selectorQuery)
         return nullptr;
+
+    NthIndexCacheScope nthIndexScope(document());
     return selectorQuery->queryFirst(*this);
 }
 
@@ -1172,6 +1176,7 @@ PassRefPtrWillBeRawPtr<StaticElementList> ContainerNode::querySelectorAll(const
     if (!selectorQuery)
         return nullptr;
 
+    NthIndexCacheScope nthIndexScope(document());
     return selectorQuery->queryAll(*this);
 }