IDL: For DOMString? attribute getters, use null string to represent null

Source/core/frame/NavigatorLanguage.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NavigatorLanguage_h
 #define NavigatorLanguage_h
 
 #include "wtf/text/AtomicString.h"
 
 namespace WebCore {
 
 class NavigatorLanguage {
 public:
     NavigatorLanguage();
 
-    AtomicString language(bool&);
+    AtomicString language();
     virtual Vector<String> languages() = 0;
     bool hasLanguagesChanged();
     void setLanguagesChanged();
 
 private:
     bool m_languagesChanged;
 };
 
 } // namespace WebCore
 
 #endif // NavigatorLanguage_h