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 b9980baebe0adb0a7e6d33e94104122ea37304eb..eaf2289d60d8e309e2fdb118bccf2224bfa8a472 100644
--- a/Source/core/dom/Node.cpp
+++ b/Source/core/dom/Node.cpp
@@ -1485,7 +1485,7 @@ 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()) {
             if (child->nodeType() == Node::COMMENT_NODE || child->nodeType() == Node::PROCESSING_INSTRUCTION_NODE)
                 continue;
             appendTextContent(child, convertBRsToNewlines, isNullString, content);