Add toHTMLHeadElement(), and use it.

Source/core/html/HTMLHeadElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  * Copyright (C) 2004, 2006, 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 19 matching lines @@
 
 class HTMLHeadElement FINAL : public HTMLElement {
 public:
     static PassRefPtr<HTMLHeadElement> create(Document&);
     static PassRefPtr<HTMLHeadElement> create(const QualifiedName&, Document&);
 
 private:
     HTMLHeadElement(const QualifiedName&, Document&);
 };
 
+inline HTMLHeadElement* toHTMLHeadElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::headTag));
+    return static_cast<HTMLHeadElement*>(node);
+}
+
 } // namespace
 
 #endif