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

Source/core/css/StyleMedia.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 22 matching lines @@
 #include "core/frame/Frame.h"
 #include "core/frame/FrameView.h"
 
 namespace WebCore {
 
 StyleMedia::StyleMedia(Frame* frame)
     : DOMWindowProperty(frame)
 {
 }
 
-String StyleMedia::type() const
+AtomicString StyleMedia::type() const
 {
     FrameView* view = m_frame ? m_frame->view() : 0;
     if (view)
         return view->mediaType();
 
-    return String();
+    return nullAtom;
 }
 
 bool StyleMedia::matchMedium(const String& query) const
 {
     if (!m_frame)
         return false;
 
     Document* document = m_frame->document();
     ASSERT(document);
     Element* documentElement = document->documentElement();
     if (!documentElement)
         return false;
 
     StyleResolver& styleResolver = document->ensureStyleResolver();
     RefPtr<RenderStyle> rootStyle = styleResolver.styleForElement(documentElement, 0 /*defaultParent*/, DisallowStyleSharing, MatchOnlyUserAgentRules);
 
     RefPtr<MediaQuerySet> media = MediaQuerySet::create();
     if (!media->set(query))
         return false;
 
     MediaQueryEvaluator screenEval(type(), m_frame, rootStyle.get());
     return screenEval.eval(media.get());
 }
 
 } // namespace WebCore