Make it clear a DocumentNameCollection can only contain HTMLElements

Source/core/html/DocumentNameCollection.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DocumentNameCollection_h
 #define DocumentNameCollection_h
 
+#include "core/html/HTMLElement.h"
 #include "core/html/HTMLNameCollection.h"
 
 namespace blink {
 
 class DocumentNameCollection FINAL : public HTMLNameCollection {
 public:
     static PassRefPtrWillBeRawPtr<DocumentNameCollection> create(ContainerNode& document, CollectionType type, const AtomicString& name)
     {
         ASSERT_UNUSED(type, type == DocumentNamedItems);
         return adoptRefWillBeNoop(new DocumentNameCollection(document, name));
     }
 
-    bool elementMatches(const Element&) const;
+    HTMLElement* item(unsigned offset) const { return toHTMLElement(HTMLNameCollection::item(offset)); }
+
+    bool elementMatches(const HTMLElement&) const;
 
 private:
     DocumentNameCollection(ContainerNode& document, const AtomicString& name);
 };
 
 DEFINE_TYPE_CASTS(DocumentNameCollection, LiveNodeListBase, collection, collection->type() == DocumentNamedItems, collection.type() == DocumentNamedItems);
 
 } // namespace blink
 
 #endif // DocumentNameCollection_h