Replace many [TreatReturnedNullStringAs=Null] with nullable DOMString

Source/core/xml/XMLHttpRequest.idl

 /*
  * Copyright (C) 2008, 2011 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
@@ skipping 51 matching lines @@
     [LogActivity, LogAllWorlds, RaisesException] void setRequestHeader(DOMString header, DOMString value);
 
     [Custom, RaisesException] void send();
 
     void abort();
 
     readonly attribute XMLHttpRequestUpload upload;
 
     // response
     [TreatReturnedNullStringAs=Undefined] DOMString getAllResponseHeaders();
-    [TreatReturnedNullStringAs=Null] DOMString getResponseHeader(DOMString header);
-    [Custom=Getter, RaisesException=Getter] readonly attribute DOMString responseText; // The custom getter implements TreatReturnedNullStringAs=Null
+    DOMString? getResponseHeader(DOMString header);
+    [Custom=Getter, RaisesException=Getter] readonly attribute DOMString? responseText;
     [RaisesException=Getter] readonly attribute Document responseXML;
 
     [RaisesException=Setter] attribute XMLHttpRequestResponseType responseType;
     [Custom=Getter, RaisesException=Getter] readonly attribute object response;
     readonly attribute DOMString responseURL;
 
     readonly attribute unsigned short status;
     readonly attribute DOMString statusText;
 
     // Extension
     void overrideMimeType(DOMString override);
 };