Sync the XHR-related interfaces with the spec

Source/core/xmlhttprequest/XMLHttpRequestEventTarget.idl

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  * Copyright (C) 2013 Samsung Electronics. All rights reserved.
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 // https://xhr.spec.whatwg.org/#xmlhttprequesteventtarget
 
 [
+    // 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;
 };