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

Source/core/css/SelectorChecker.h

Index: Source/core/css/SelectorChecker.h
diff --git a/Source/core/css/SelectorChecker.h b/Source/core/css/SelectorChecker.h
index c1f7fd148e9d59bb3e9fb4d6eaa404104c818a10..7a56569667beb767e2d0d7cbc07357f7c1bb75fc 100644
--- a/Source/core/css/SelectorChecker.h
+++ b/Source/core/css/SelectorChecker.h
@@ -55,7 +55,7 @@ public:
 
     struct SelectorCheckingContext {
         // Initial selector constructor
-        SelectorCheckingContext(const CSSSelector* selector, Element* element, VisitedMatchType visitedMatchType)
+        SelectorCheckingContext(const CSSSelector* selector, Element* element, VisitedMatchType visitedMatchType, int childIndex = 0)
             : selector(selector)
             , element(element)
             , scope(0)
@@ -68,6 +68,7 @@ public:
             , hasScrollbarPseudo(false)
             , hasSelectionPseudo(false)
             , behaviorAtBoundary(DoesNotCrossBoundary)
+            , childIndex(childIndex)
         { }
 
         const CSSSelector* selector;
@@ -82,6 +83,7 @@ public:
         bool hasScrollbarPseudo;
         bool hasSelectionPseudo;
         BehaviorAtBoundary behaviorAtBoundary;
+        int childIndex;
     };
 
     template<typename SiblingTraversalStrategy>