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 542d850c8d33077590073439f6443f08da9b061b..cb8f31309f048e6ccc3c01fc1c60d479d25a7a19 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -87,6 +87,7 @@
 #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/ScriptRunner.h"
 #include "core/dom/ScriptedAnimationController.h"
@@ -1769,6 +1770,8 @@ void Document::updateStyle(StyleRecalcChange change)
     HTMLFrameOwnerElement::UpdateSuspendScope suspendWidgetHierarchyUpdates;
     m_lifecycle.advanceTo(DocumentLifecycle::InStyleRecalc);
 
+    NthIndexCache nthIndexCache(*this);
+
     if (styleChangeType() >= SubtreeStyleChange)
         change = Force;