Make calls to AtomicString(const String&) explicit in web/

Source/core/dom/Element.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2013 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 357 matching lines @@
 
     virtual void accessKeyAction(bool /*sendToAnyEvent*/) { }
 
     virtual bool isURLAttribute(const Attribute&) const { return false; }
     virtual bool isHTMLContentAttribute(const Attribute&) const { return false; }
 
     KURL getURLAttribute(const QualifiedName&) const;
     KURL getNonEmptyURLAttribute(const QualifiedName&) const;
 
     virtual const AtomicString imageSourceURL() const;
-    virtual String target() const { return String(); }
+    virtual AtomicString target() const { return nullAtom; }
     virtual Image* imageContents() { return 0; }
 
     virtual void focus(bool restorePreviousSelection = true, FocusDirection = FocusDirectionNone);
     virtual void updateFocusAppearance(bool restorePreviousSelection);
     virtual void blur();
     // Whether this element can receive focus at all. Most elements are not
     // focusable but some elements, such as form controls and links, are. Unlike
     // rendererIsFocusable(), this method may be called when layout is not up to
     // date, so it must not use the renderer to determine focusability.
     virtual bool supportsFocus() const;
@@ skipping 507 matching lines @@
 }
 
 inline bool isShadowHost(const Element* element)
 {
     return element && element->shadow();
 }
 
 } // namespace
 
 #endif