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

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index 51a3c60befaffd8c8722eedee272a4a7ce04c092..e0e2ed1474edf80d27f6c43a66112a40d882ec54 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -128,6 +128,7 @@ class MediaQueryListListener;
 class MediaQueryMatcher;
 class NodeFilter;
 class NodeIterator;
+class NthIndexCache;
 class Page;
 class PlatformMouseEvent;
 class ProcessingInstruction;
@@ -247,6 +248,7 @@ public:
     virtual bool canContainRangeEndPoint() const override { return true; }
 
     SelectorQueryCache& selectorQueryCache();
+    NthIndexCache* nthIndexCache();
 
     // Focus Management.
     Element* activeElement() const;
@@ -1080,6 +1082,7 @@ protected:
 
 private:
     friend class IgnoreDestructiveWriteCountIncrementer;
+    friend class NthIndexCacheScope;
 
     bool isDocumentFragment() const = delete; // This will catch anyone doing an unnecessary check.
     bool isDocumentNode() const = delete; // This will catch anyone doing an unnecessary check.
@@ -1172,6 +1175,9 @@ private:
 
     void updateElementOpacity(const AtomicString& cssSelector, double opacity);
 
+    bool useNthIndexCache() const { return m_useNthIndexCache; }
+    void setUseNthIndexCache(bool);
+
     DocumentLifecycle m_lifecycle;
 
     bool m_hasNodesWithPlaceholderStyle;
@@ -1317,6 +1323,7 @@ private:
     WillBeHeapHashMap<String, RefPtrWillBeMember<HTMLCanvasElement>> m_cssCanvasElements;
 
     OwnPtr<SelectorQueryCache> m_selectorQueryCache;
+    OwnPtrWillBeMember<NthIndexCache> m_nthIndexCache;
 
     bool m_useSecureKeyboardEntryWhenActive;
 
@@ -1397,6 +1404,8 @@ private:
     ParserSynchronizationPolicy m_parserSyncPolicy;
 
     OriginsUsingFeatures::Value m_originsUsingFeaturesValue;
+
+    bool m_useNthIndexCache;
 };
 
 inline bool Document::shouldOverrideLegacyDescription(ViewportDescription::Type origin)