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

Source/core/html/parser/HTMLSrcsetParser.h

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 32 matching lines @@
     }
 
     ImageCandidate(const String& source, unsigned start, unsigned length, float scaleFactor)
         : m_string(source.createView(start, length))
         , m_scaleFactor(scaleFactor)
     {
     }
 
     String toString() const
     {
-        return m_string.toString();
+        return String(m_string.toString());
+    }
+
+    AtomicString toAtomicString() const
+    {
+        return AtomicString(m_string.toString());
     }
 
     inline float scaleFactor() const
     {
         return m_scaleFactor;
     }
 
     inline bool isEmpty() const
     {
         return m_string.isEmpty();
     }
 
 private:
     StringView m_string;
     float m_scaleFactor;
 };
 
 ImageCandidate bestFitSourceForSrcsetAttribute(float deviceScaleFactor, const String& srcsetAttribute);
 
 ImageCandidate bestFitSourceForImageAttributes(float deviceScaleFactor, const String& srcAttribute, const String& srcsetAttribute);
 
 String bestFitSourceForImageAttributes(float deviceScaleFactor, const String& srcAttribute, ImageCandidate& srcsetImageCandidate);
 
 }
 
 #endif