Update HTTPHeaderMap wrappers to use AtomicString type for header values

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 3725 matching lines @@
         return;
     }
 
     KURL cookieURL = this->cookieURL();
     if (cookieURL.isEmpty())
         return;
 
     setCookies(this, cookieURL, value);
 }
 
-String Document::referrer() const
+AtomicString Document::referrer() const
 {
     if (loader())
         return loader()->request().httpReferrer();
-    return String();
+    return nullAtom;
 }
 
 String Document::domain() const
 {
     return securityOrigin()->domain();
 }
 
 void Document::setDomain(const String& newDomain, ExceptionState& exceptionState)
 {
     if (isSandboxed(SandboxDocumentDomain)) {
@@ skipping 1452 matching lines @@
 }
 
 FastTextAutosizer* Document::fastTextAutosizer()
 {
     if (!m_fastTextAutosizer && RuntimeEnabledFeatures::fastTextAutosizingEnabled())
         m_fastTextAutosizer = FastTextAutosizer::create(this);
     return m_fastTextAutosizer.get();
 }
 
 } // namespace WebCore