Use new is*Element() helper functions more in editing code

Source/core/editing/MarkupAccumulator.cpp

Index: Source/core/editing/MarkupAccumulator.cpp
diff --git a/Source/core/editing/MarkupAccumulator.cpp b/Source/core/editing/MarkupAccumulator.cpp
index daa7040d702fab0a5dc5fece4a106c8cde8422b4..0dd6adf8c8282d0b840bcda4951a3b8a1a8310e8 100644
--- a/Source/core/editing/MarkupAccumulator.cpp
+++ b/Source/core/editing/MarkupAccumulator.cpp
@@ -139,7 +139,7 @@ void MarkupAccumulator::serializeNodesWithNamespaces(Node& targetNode, EChildren
         appendStartTag(targetNode, &namespaceHash);
 
     if (!(serializeAsHTMLDocument(targetNode) && elementCannotHaveEndTag(targetNode))) {
-        Node* current = targetNode.hasTagName(templateTag) ? toHTMLTemplateElement(targetNode).content()->firstChild() : targetNode.firstChild();
+        Node* current = isHTMLTemplateElement(targetNode) ? toHTMLTemplateElement(targetNode).content()->firstChild() : targetNode.firstChild();
         for ( ; current; current = current->nextSibling())
             serializeNodesWithNamespaces(*current, IncludeNode, &namespaceHash, tagNamesToSkip);
     }