Avoid N^2 walk placing renderers when building the render tree

Source/core/css/resolver/StyleResolver.h

Index: Source/core/css/resolver/StyleResolver.h
diff --git a/Source/core/css/resolver/StyleResolver.h b/Source/core/css/resolver/StyleResolver.h
index d3eb47cccee2f8eaf29eca3710b0b34e12c29f21..4baf00a0a6fe20370e000c8830716f34ca630a6e 100644
--- a/Source/core/css/resolver/StyleResolver.h
+++ b/Source/core/css/resolver/StyleResolver.h
@@ -146,7 +146,13 @@ public:
     void popParentShadowRoot(const ShadowRoot*);
 
     PassRefPtr<RenderStyle> styleForElement(Element*, RenderStyle* parentStyle = 0, StyleSharingBehavior = AllowStyleSharing,
-        RuleMatchingBehavior = MatchAllRules, RenderRegion* regionForStyling = 0);
+        RuleMatchingBehavior = MatchAllRules, RenderRegion* regionForStyling = 0, int childIndex = 0);
+
+    // childIndex's origin is 1, and avoids unnecessary tree walks to resolve nth/nth-last selectors.
+    PassRefPtr<RenderStyle> styleForElement(Element* element, int childIndex)
+    {
+        return styleForElement(element, 0, AllowStyleSharing, MatchAllRules, 0, childIndex);
+    }
 
     void keyframeStylesForAnimation(Element*, const RenderStyle*, KeyframeList&);
 
@@ -187,7 +193,7 @@ public:
     }
 
 private:
-    void initElement(Element*);
+    void initElement(Element*, int childIndex = 0);
     RenderStyle* locateSharedStyle();
     bool styleSharingCandidateMatchesRuleSet(RuleSet*);
     Node* locateCousinList(Element* parent, unsigned& visitedNodeCount) const;