Avoid calling slower Node::firstChild() / Node::lastChild() when possible

Source/core/dom/ContainerNode.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 90 matching lines @@
     willBeDeletedFromDocument();
     removeDetachedChildren();
 #endif
 }
 
 bool ContainerNode::isChildTypeAllowed(const Node& child) const
 {
     if (!child.isDocumentFragment())
         return childTypeAllowed(child.nodeType());
 
-    for (Node* node = child.firstChild(); node; node = node->nextSibling()) {
+    for (Node* node = toDocumentFragment(child).firstChild(); node; node = node->nextSibling()) {
         if (!childTypeAllowed(node->nodeType()))
             return false;
     }
     return true;
 }
 
 bool ContainerNode::containsConsideringHostElements(const Node& newChild) const
 {
     if (isInShadowTree() || document().isTemplateDocument())
         return newChild.containsIncludingHostElements(*this);
@@ skipping 1156 matching lines @@
         return true;
 
     if (node->isElementNode() && toElement(node)->shadow())
         return true;
 
     return false;
 }
 #endif
 
 } // namespace WebCore