Add Client Attribute to FetchEvent- Blink Side.

Source/modules/serviceworkers/FetchEvent.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://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#fetch-event-interface
 [
     Constructor(DOMString type, optional FetchEventInit eventInitDict),
     RuntimeEnabled=ServiceWorkerOnFetch,
     Exposed=ServiceWorker,
     TypeChecking=Interface,
 ] interface FetchEvent : ExtendableEvent {
     readonly attribute Request request;
+    readonly attribute Client client;
     readonly attribute boolean isReload;
 
     [CallWith=ScriptState, RaisesException] void respondWith(any value);
 };