Use VisibilityState enum and move WebPageVisibilityState to public/platform.

Source/modules/serviceworkers/WindowClient.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/#context-frame-type-enum
 enum ContextFrameType {
     "top-level",
     "nested",
     "auxiliary",
     "none"
 };
 
+// http://www.w3.org/TR/page-visibility/#VisibilityState
+enum VisibilityState { "hidden", "visible", "prerender", "unloaded" };
+
 // https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#window-client-interface
 [
     Exposed=ServiceWorker,
     RuntimeEnabled=ServiceWorkerClientAttributes,
     GarbageCollected,
     TypeChecking=Interface,
     ImplementedAs=ServiceWorkerWindowClient,
 ] interface WindowClient : Client {
-    readonly attribute DOMString visibilityState;
+    readonly attribute VisibilityState visibilityState;
     readonly attribute boolean focused;
     readonly attribute ContextFrameType frameType;
     [CallWith=ScriptState] Promise<boolean> focus();
 };