Remove support for the obsolete <isindex> tag.

Source/core/html/parser/HTMLStackItem.h

 /*
  * Copyright (C) 2012 Company 100, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 154 matching lines @@
             || tagName == HTMLNames::framesetTag
             || isNumberedHeaderElement()
             || tagName == HTMLNames::headTag
             || tagName == HTMLNames::headerTag
             || tagName == HTMLNames::hgroupTag
             || tagName == HTMLNames::hrTag
             || tagName == HTMLNames::htmlTag
             || tagName == HTMLNames::iframeTag
             || tagName == HTMLNames::imgTag
             || tagName == HTMLNames::inputTag
-            || tagName == HTMLNames::isindexTag
             || tagName == HTMLNames::liTag
             || tagName == HTMLNames::linkTag
             || tagName == HTMLNames::listingTag
             || tagName == HTMLNames::mainTag
             || tagName == HTMLNames::marqueeTag
             || tagName == HTMLNames::menuTag
             || tagName == HTMLNames::metaTag
             || tagName == HTMLNames::navTag
             || tagName == HTMLNames::noembedTag
             || tagName == HTMLNames::noframesTag
@@ skipping 51 matching lines @@
 
     AtomicString m_tokenLocalName;
     Vector<Attribute> m_tokenAttributes;
     AtomicString m_namespaceURI;
     bool m_isDocumentFragmentNode;
 };
 
 } // namespace WebCore
 
 #endif // HTMLStackItem_h