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

Source/core/dom/Node.cpp

Index: Source/core/dom/Node.cpp
diff --git a/Source/core/dom/Node.cpp b/Source/core/dom/Node.cpp
index 00d15ffe33f87e86972771db485079ebb87c8f66..38179ffa1d15af2f96a63e8847bd0eb7f08e4af7 100644
--- a/Source/core/dom/Node.cpp
+++ b/Source/core/dom/Node.cpp
@@ -1483,7 +1483,7 @@ static void appendTextContent(const Node* node, bool convertBRsToNewlines, bool&
     case Node::ATTRIBUTE_NODE:
     case Node::DOCUMENT_FRAGMENT_NODE:
         isNullString = false;
-        for (Node* child = node->firstChild(); child; child = child->nextSibling()) {
+        for (Node* child = toContainerNode(node)->firstChild(); child; child = child->nextSibling()) {
             Node::NodeType childNodeType = child->nodeType();
             if (childNodeType == Node::COMMENT_NODE || childNodeType == Node::PROCESSING_INSTRUCTION_NODE)
                 continue;