Update HTTPHeaderMap wrappers to use AtomicString type for header values

Source/core/fetch/ScriptResource.cpp

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller (mueller@kde.org)
     Copyright (C) 2002 Waldo Bastian (bastian@kde.org)
     Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
     Copyright (C) 2004, 2005, 2006, 2007, 2008 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
@@ skipping 42 matching lines @@
 void ScriptResource::setEncoding(const String& chs)
 {
     m_decoder->setEncoding(chs, TextResourceDecoder::EncodingFromHTTPHeader);
 }
 
 String ScriptResource::encoding() const
 {
     return m_decoder->encoding().name();
 }
 
-String ScriptResource::mimeType() const
+AtomicString ScriptResource::mimeType() const
 {
     return extractMIMETypeFromMediaType(m_response.httpHeaderField("Content-Type")).lower();
 }
 
 const String& ScriptResource::script()
 {
     ASSERT(!isPurgeable());
     ASSERT(isLoaded());
 
     if (!m_script && m_data) {
         String script = m_decoder->decode(m_data->data(), encodedSize());
         script.append(m_decoder->flush());
         m_data.clear();
         // We lie a it here and claim that script counts as encoded data (even though it's really decoded data).
         // That's because the MemoryCache thinks that it can clear out decoded data by calling destroyDecodedData(),
         // but we can't destroy script in destroyDecodedData because that's our only copy of the data!
         setEncodedSize(script.sizeInBytes());
         m_script = script;
     }
 
     return m_script.string();
 }
 
 bool ScriptResource::mimeTypeAllowedByNosniff() const
 {
     return parseContentTypeOptionsHeader(m_response.httpHeaderField("X-Content-Type-Options")) != ContentTypeOptionsNosniff || MIMETypeRegistry::isSupportedJavaScriptMIMEType(mimeType());
 }
 
 } // namespace WebCore