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

Source/core/css/StyleMedia.cpp

Index: Source/core/css/StyleMedia.cpp
diff --git a/Source/core/css/StyleMedia.cpp b/Source/core/css/StyleMedia.cpp
index d542f44255f3787578ef5504aa4ed83b183f68b9..791ef27328f2a10ca761be1c959cd9ce7423b5a5 100644
--- a/Source/core/css/StyleMedia.cpp
+++ b/Source/core/css/StyleMedia.cpp
@@ -40,13 +40,13 @@ StyleMedia::StyleMedia(Frame* 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