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

Source/core/css/SelectorChecker.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 40 matching lines @@
     enum BehaviorAtBoundary {
         DoesNotCrossBoundary = 0,
         CrossesBoundary = 1,
         StaysWithinTreeScope = 2,
         BoundaryBehaviorMask = 3, // 2bit for boundary behavior
         ScopeContainsLastMatchedElement = 4,
     };
 
     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)
             , visitedMatchType(visitedMatchType)
             , pseudoId(NOPSEUDO)
             , elementStyle(0)
             , scrollbar(0)
             , scrollbarPart(NoPart)
             , isSubSelector(false)
             , hasScrollbarPseudo(false)
             , hasSelectionPseudo(false)
             , behaviorAtBoundary(DoesNotCrossBoundary)
+            , childIndex(childIndex)
         { }
 
         const CSSSelector* selector;
         Element* element;
         const ContainerNode* scope;
         VisitedMatchType visitedMatchType;
         PseudoId pseudoId;
         RenderStyle* elementStyle;
         RenderScrollbar* scrollbar;
         ScrollbarPart scrollbarPart;
         bool isSubSelector;
         bool hasScrollbarPseudo;
         bool hasSelectionPseudo;
         BehaviorAtBoundary behaviorAtBoundary;
+        int childIndex;
     };
 
     template<typename SiblingTraversalStrategy>
     Match match(const SelectorCheckingContext&, PseudoId&, const SiblingTraversalStrategy&) const;
 
     template<typename SiblingTraversalStrategy>
     Match matchForShadowDistributed(const Element*, const SiblingTraversalStrategy&, PseudoId&, SelectorCheckingContext& nextContext) const;
 
     template<typename SiblingTraversalStrategy>
     bool checkOne(const SelectorCheckingContext&, const SiblingTraversalStrategy&) const;
@@ skipping 51 matching lines @@
         const Attribute* attribute = element->attributeItem(i);
         if (attribute->matches(selectorAttributeName) && (!value || attribute->value().impl() == value))
             return true;
     }
     return false;
 }
 
 }
 
 #endif