Have HashMap<KeyType, AtomicString>::get() return a const reference

Source/core/loader/FrameLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008 Alp Toker <alp@atoker.com>
  * Copyright (C) Research In Motion Limited 2009. All rights reserved.
  * Copyright (C) 2011 Kris Jordan <krisjordan@gmail.com>
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ skipping 337 matching lines @@
 
     m_frame->document()->initContentSecurityPolicy(m_documentLoader ? ContentSecurityPolicyResponseHeaders(m_documentLoader->response()) : ContentSecurityPolicyResponseHeaders());
 
     Settings* settings = m_frame->document()->settings();
     if (settings) {
         m_frame->document()->fetcher()->setImagesEnabled(settings->areImagesEnabled());
         m_frame->document()->fetcher()->setAutoLoadImages(settings->loadsImagesAutomatically());
     }
 
     if (m_documentLoader) {
-        String dnsPrefetchControl = m_documentLoader->response().httpHeaderField("X-DNS-Prefetch-Control");
+        const AtomicString& dnsPrefetchControl = m_documentLoader->response().httpHeaderField("X-DNS-Prefetch-Control");
         if (!dnsPrefetchControl.isEmpty())
             m_frame->document()->parseDNSPrefetchControlHeader(dnsPrefetchControl);
 
         String headerContentLanguage = m_documentLoader->response().httpHeaderField("Content-Language");
         if (!headerContentLanguage.isEmpty()) {
             size_t commaIndex = headerContentLanguage.find(',');
             headerContentLanguage.truncate(commaIndex); // kNotFound == -1 == don't truncate
             headerContentLanguage = headerContentLanguage.stripWhiteSpace(isHTMLSpace<UChar>);
             if (!headerContentLanguage.isEmpty())
                 m_frame->document()->setContentLanguage(headerContentLanguage);
@@ skipping 1166 matching lines @@
 {
     SandboxFlags flags = m_forcedSandboxFlags;
     if (Frame* parentFrame = m_frame->tree().parent())
         flags |= parentFrame->document()->sandboxFlags();
     if (HTMLFrameOwnerElement* ownerElement = m_frame->ownerElement())
         flags |= ownerElement->sandboxFlags();
     return flags;
 }
 
 } // namespace WebCore