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

Source/core/dom/Document.h

 /*
  * 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, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 708 matching lines @@
     String title() const { return m_title; }
     void setTitle(const String&);
 
     Element* titleElement() const { return m_titleElement.get(); }
     void setTitleElement(const String& title, Element* titleElement);
     void removeTitle(Element* titleElement);
 
     String cookie(ExceptionState&) const;
     void setCookie(const String&, ExceptionState&);
 
-    AtomicString referrer() const;
+    const AtomicString& referrer() const;
 
     String domain() const;
     void setDomain(const String& newDomain, ExceptionState&);
 
     String lastModified() const;
 
     // The cookieURL is used to query the cookie database for this document's
     // cookies. For example, if the cookie URL is http://example.com, we'll
     // use the non-Secure cookies for example.com when computing
     // document.cookie.
@@ skipping 621 matching lines @@
 inline bool Node::isDocumentNode() const
 {
     return this == documentInternal();
 }
 
 Node* eventTargetNodeForDocument(Document*);
 
 } // namespace WebCore
 
 #endif // Document_h