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

Source/core/dom/ContainerNode.cpp

Index: Source/core/dom/ContainerNode.cpp
diff --git a/Source/core/dom/ContainerNode.cpp b/Source/core/dom/ContainerNode.cpp
index 116d4763b1d45869f3c32854046cf486c73ebbad..fb4fb4858eb13c6623c3c35091fcbe6aeabc52e9 100644
--- a/Source/core/dom/ContainerNode.cpp
+++ b/Source/core/dom/ContainerNode.cpp
@@ -108,7 +108,7 @@ 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;
     }