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

third_party/WebKit/Source/modules/fetch/Headers.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://fetch.spec.whatwg.org/#typedefdef-headersinit
+
+// The actual typedef in the spec is
+//    (sequence<sequence<ByteString>> or record<ByteString,ByteString>)
+// which also implicitly includes Headers since it is iterable.
+// See http://crbug.com/690428.
+typedef (sequence<sequence<ByteString>> or record<ByteString, ByteString> or Headers) HeadersInit;
+
 // https://fetch.spec.whatwg.org/#headers-class
 
-typedef Dictionary OpenEndedDictionary;
 [
-    Constructor,
-    Constructor(Headers input),
-    Constructor(OpenEndedDictionary input),
-    Constructor(sequence<sequence<ByteString>> input),
+    Constructor(optional HeadersInit init),
     Exposed=(Window,Worker),
     RaisesException=Constructor,
 ] interface Headers {
     [RaisesException] void append(ByteString name, ByteString value);
     [ImplementedAs=remove, RaisesException] void delete(ByteString key);
     [RaisesException] ByteString? get(ByteString key);
     [RaisesException, Measure] sequence<ByteString> getAll(ByteString name);
     [RaisesException] boolean has(ByteString key);
     [RaisesException] void set(ByteString key, ByteString value);
     iterable<ByteString, ByteString>;
 };