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

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 7d6e736cb59209b64da43fe1468295815357e35c..a12a898b841342e00bd8a4bb1f3af71a393efdb9 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -1671,7 +1671,7 @@ void Document::recalcStyle(StyleChange change)
                 renderer()->setStyle(documentStyle.release());
         }
 
-        for (Node* n = firstChild(); n; n = n->nextSibling()) {
+        for (Node* n = lastChild(); n; n = n->previousSibling()) {
             if (!n->isElementNode())
                 continue;
             Element* element = toElement(n);