Sync the XHR-related interfaces with the spec

Source/core/xmlhttprequest/XMLHttpRequestUpload.idl

 /*
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+// https://xhr.spec.whatwg.org/#xmlhttprequestupload
+
+// TODO(philipj): The spec has Exposed=(Window,Worker)
+// https://github.com/whatwg/xhr/issues/19
 interface XMLHttpRequestUpload : XMLHttpRequestEventTarget {
 };
-