Fetch: Make Headers' constructor match the current spec IDL.

third_party/WebKit/Source/modules/fetch/ResponseInit.idl

 // Copyright 2017 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://fetch.spec.whatwg.org/#responseinit
 
-// The actual typedef in the spec is
-//    (sequence<sequence<ByteString>> or record<ByteString,ByteString>)
-// which also implicitly includes Headers since it is iterable.
-// Blink's WebIDL code does not support record<K,V> yet, so we have to make do
-// with the (Dictionary or Headers) part instead.
-// See http://crbug.com/685754.
-typedef (sequence<sequence<ByteString>> or Dictionary or Headers) HeadersInit;
-
 dictionary ResponseInit {
     unsigned short status = 200;
     ByteString statusText = "OK";
     HeadersInit headers;
 };