Introduce Response.redirect attribute and add LayoutTest.

third_party/WebKit/Source/modules/fetch/Response.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/#response-class
 
 enum ResponseType { "basic", "cors", "default", "error", "opaque", "opaqueredirect" };
 
 [
     ActiveScriptWrappable,
     // TODO(yhirano): We use "any" for body because the IDL processor doesn't
     // recognize ReadableStream implemented with V8 extras. Fix it.
     Constructor(optional any body, optional Dictionary responseInitDict),
     ConstructorCallWith=ScriptState,
     DependentLifetime,
     Exposed=(Window,Worker),
     RaisesException=Constructor,
 ] interface Response {
     [CallWith=ExecutionContext] static Response error();
     [CallWith=ExecutionContext, RaisesException] static Response redirect(USVString url, optional unsigned short status = 302);
     readonly attribute ResponseType type;
     readonly attribute USVString url;
+    readonly attribute boolean redirected;
     readonly attribute unsigned short status;
     readonly attribute boolean ok;
     readonly attribute ByteString statusText;
     readonly attribute Headers headers;
 
     [RaisesException, CallWith=ScriptState] Response clone();
 
     [CallWith=ScriptState, ImplementedAs=bodyWithUseCounter] readonly attribute any body;
 };
 
 Response implements Body;