Update DOMTokenList::value() to return a const reference

Source/core/html/HTMLLinkElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google 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
  * version 2 of the License, or (at your option) any later version.
@@ skipping 118 matching lines @@
     String typeValue() const { return m_type; }
     const LinkRelAttribute& relAttribute() const { return m_relAttribute; }
 
     virtual AtomicString target() const OVERRIDE;
 
     const AtomicString& type() const;
 
     IconType iconType() const;
 
     // the icon size string as parsed from the HTML attribute
-    String iconSizes() const;
+    const AtomicString& iconSizes() const;
 
     CSSStyleSheet* sheet() const { return linkStyle() ? linkStyle()->sheet() : 0; }
     Document* import() const;
 
     bool styleSheetIsLoading() const;
 
     bool isDisabled() const { return linkStyle() && linkStyle()->isDisabled(); }
     bool isEnabledViaScript() const { return linkStyle() && linkStyle()->isEnabledViaScript(); }
     DOMSettableTokenList* sizes() const;
 
@@ skipping 51 matching lines @@
     bool m_createdByParser;
     bool m_isInShadowTree;
     int m_beforeLoadRecurseCount;
 };
 
 DEFINE_NODE_TYPE_CASTS(HTMLLinkElement, hasTagName(HTMLNames::linkTag));
 
 } //namespace
 
 #endif