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

Source/core/css/resolver/StyleResolverState.h

Index: Source/core/css/resolver/StyleResolverState.h
diff --git a/Source/core/css/resolver/StyleResolverState.h b/Source/core/css/resolver/StyleResolverState.h
index 2c87baed4f633f6fcf50e8ff4d22548059c004d6..728f85c0a0dcfdd8dabd6bdd359c660e413fccd3 100644
--- a/Source/core/css/resolver/StyleResolverState.h
+++ b/Source/core/css/resolver/StyleResolverState.h
@@ -47,6 +47,7 @@ WTF_MAKE_NONCOPYABLE(StyleResolverState);
 public:
     StyleResolverState()
     : m_element(0)
+    , m_childIndex(0)
     , m_styledElement(0)
     , m_parentNode(0)
     , m_parentStyle(0)
@@ -64,7 +65,7 @@ public:
     , m_backgroundData(BackgroundFillLayer) { }
 
     public:
-    void initElement(Element*);
+    void initElement(Element*, int childIndex);
     void initForStyleResolve(Document*, Element*, RenderStyle* parentStyle = 0, RenderRegion* regionForStyling = 0);
     void clear();
 
@@ -72,6 +73,7 @@ public:
 
     Document* document() const { return m_element->document(); }
     Element* element() const { return m_element; }
+    int childIndex() const { return m_childIndex; }
     Element* styledElement() const { return m_styledElement; }
     void setStyle(PassRefPtr<RenderStyle> style) { m_style = style; }
     RenderStyle* style() const { return m_style.get(); }
@@ -120,6 +122,7 @@ public:
 
 private:
     Element* m_element;
+    int m_childIndex;
     RefPtr<RenderStyle> m_style;
     Element* m_styledElement;
     ContainerNode* m_parentNode;