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

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 0e83a6fc8b4c3f1f9d4761ba9695a15ff178e575..578ec9468a4d459df5fffc5e037c490a128ce5bd 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -86,6 +86,7 @@
 #include "core/dom/NodeRenderingTraversal.h"
 #include "core/dom/NodeTraversal.h"
 #include "core/dom/NodeWithIndex.h"
+#include "core/dom/NthIndexCache.h"
 #include "core/dom/ProcessingInstruction.h"
 #include "core/dom/RequestAnimationFrameCallback.h"
 #include "core/dom/ScriptRunner.h"
@@ -1773,6 +1774,8 @@ void Document::updateStyle(StyleRecalcChange change)
     HTMLFrameOwnerElement::UpdateSuspendScope suspendWidgetHierarchyUpdates;
     m_lifecycle.advanceTo(DocumentLifecycle::InStyleRecalc);
 
+    NthIndexCache nthIndexCache(*this);
+
     if (styleChangeType() >= SubtreeStyleChange)
         change = Force;