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

Source/core/html/track/vtt/VTTParser.h

 /*
  * Copyright (C) 2011 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 119 matching lines @@
     void createNewCue();
     void resetCueValues();
 
     void collectMetadataHeader(const String&);
     void createNewRegion(const String& headerValue);
 
     static void skipWhiteSpace(const String&, unsigned*);
 
     BufferedLineReader m_lineReader;
     OwnPtr<TextResourceDecoder> m_decoder;
-    String m_currentId;
+    AtomicString m_currentId;
     double m_currentStartTime;
     double m_currentEndTime;
     StringBuilder m_currentContent;
     String m_currentSettings;
 
     VTTParserClient* m_client;
 
     Vector<RefPtr<VTTCue> > m_cuelist;
 
     Vector<RefPtr<VTTRegion> > m_regionList;
 };
 
 } // namespace WebCore
 
 #endif