Update the visibility of XHR-related interfaces to match spec

Source/core/xmlhttprequest/XMLHttpRequestEventTarget.idl

 /*
  * Copyright (C) 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Samsung Electronics. 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 10 matching lines @@
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // https://xhr.spec.whatwg.org/#xmlhttprequesteventtarget
 
 [
     GarbageCollected,
-    // TODO(philipj): The spec has Exposed=(Window,Worker)
-    // https://github.com/whatwg/xhr/issues/19
     Exposed=(Window,DedicatedWorker,SharedWorker)
 ] interface XMLHttpRequestEventTarget : EventTarget {
     // event handlers
     attribute EventHandler onloadstart;
     attribute EventHandler onprogress;
     attribute EventHandler onabort;
     attribute EventHandler onerror;
     attribute EventHandler onload;
     attribute EventHandler ontimeout;
     attribute EventHandler onloadend;
 };