Add toHTMLMetaElement(), and use it.

Source/core/html/HTMLMetaElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 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 27 matching lines @@
 
 private:
     HTMLMetaElement(const QualifiedName&, Document&);
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) OVERRIDE;
     virtual InsertionNotificationRequest insertedInto(ContainerNode*) OVERRIDE;
 
     void process();
 };
 
-} //namespace
+inline HTMLMetaElement* toHTMLMetaElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::metaTag));
+    return static_cast<HTMLMetaElement*>(node);
+}
+
+} // namespace WebCore
 
 #endif