Add initial support for [Exposed=xx] extended IDL attribute

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 22 matching lines @@
     "document",
     "json",
     "text",
     "stream"
 };
 
 [
     WillBeGarbageCollected,
     ActiveDOMObject,
     CustomConstructor(optional XMLHttpRequestOptions options),
-    GlobalContext=Window&WorkerGlobalScope,
+    Exposed=Window&Worker
 ] interface XMLHttpRequest : XMLHttpRequestEventTarget {
     // event handler attributes
     attribute EventHandler onreadystatechange;
 
     // state
     const unsigned short UNSENT = 0;
     const unsigned short OPENED = 1;
     const unsigned short HEADERS_RECEIVED = 2;
     const unsigned short LOADING = 3;
     const unsigned short DONE = 4;
@@ skipping 21 matching lines @@
 
     [RaisesException=Setter] attribute XMLHttpRequestResponseType responseType;
     [Custom=Getter, RaisesException=Getter] readonly attribute object response;
 
     readonly attribute unsigned short status;
     readonly attribute DOMString statusText;
 
     // Extension
     void overrideMimeType(DOMString override);
 };