Make AutocompleteErrorEventInit.reason be of type AutocompleteErrorReason (not DOMString)

Source/core/events/AutocompleteErrorEventInit.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // https://html.spec.whatwg.org/multipage/forms.html#the-autocompleteerrorevent-interface
 
 dictionary AutocompleteErrorEventInit : EventInit {
-    // FIXME: According to the spec, "reason" should be AutocompleteErrorReason.
-    DOMString reason;
+    AutocompleteErrorReason reason;
 };