Make it obvious an HTMLFormControlsCollection can only contain HTMLElements

Source/core/html/HTMLFormElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 38 matching lines @@
 class HTMLFormElement FINAL : public HTMLElement {
 public:
     static PassRefPtrWillBeRawPtr<HTMLFormElement> create(Document&);
     virtual ~HTMLFormElement();
     virtual void trace(Visitor*) OVERRIDE;
 
     PassRefPtrWillBeRawPtr<HTMLFormControlsCollection> elements();
     void getNamedElements(const AtomicString&, WillBeHeapVector<RefPtrWillBeMember<Element> >&);
 
     unsigned length() const;
-    Element* item(unsigned index);
+    HTMLElement* item(unsigned index);
 
     String enctype() const { return m_attributes.encodingType(); }
     void setEnctype(const AtomicString&);
 
     String encoding() const { return m_attributes.encodingType(); }
     void setEncoding(const AtomicString& value) { setEnctype(value); }
 
     bool shouldAutocomplete() const;
 
     void associate(FormAssociatedElement&);
@@ skipping 114 matching lines @@
     bool m_isInResetFunction;
 
     bool m_wasDemoted;
 
     OwnPtrWillBeMember<GenericEventQueue> m_pendingAutocompleteEventsQueue;
 };
 
 } // namespace blink
 
 #endif // HTMLFormElement_h