Move Fetch API releted code to modules/fetch.

Source/modules/serviceworkers/Request.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.
-
-// http://fetch.spec.whatwg.org/#request-class
-
-typedef (Request or USVString) RequestInfo;
-
-enum RequestMode { "same-origin", "no-cors", "cors" };
-enum RequestCredentials { "omit", "same-origin", "include" };
-
-[
-    Constructor(RequestInfo input, optional Dictionary requestInitDict),
-    ConstructorCallWith=ExecutionContext,
-    Exposed(Window GlobalFetch, DedicatedWorker GlobalFetch, SharedWorker GlobalFetch, ServiceWorker ServiceWorker),
-    RaisesException=Constructor,
-    ActiveDOMObject,
-    GarbageCollected,
-    TypeChecking=Interface,
-] interface Request {
-    readonly attribute ByteString method;
-    readonly attribute USVString url;
-    readonly attribute Headers headers;
-
-    readonly attribute DOMString referrer;
-    readonly attribute RequestMode mode;
-    readonly attribute RequestCredentials credentials;
-
-    [RaisesException] Request clone();
-
-    // FIXME: Implement the following:
-    // readonly attribute RequestContext context;
-};
-
-Request implements Body;