Implement and ship PushSubscription.expirationTime

third_party/WebKit/Source/modules/push_messaging/PushSubscription.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://w3c.github.io/push-api/#pushsubscription-interface
 
 enum PushEncryptionKeyName {
     "p256dh",
     "auth"
 };
 
 [
     Exposed=(Window,Worker),
     RuntimeEnabled=PushMessaging,
 ] interface PushSubscription {
     readonly attribute USVString endpoint;
+    readonly attribute DOMTimeStamp? expirationTime;
     [SameObject] readonly attribute PushSubscriptionOptions options;
 
     ArrayBuffer? getKey(PushEncryptionKeyName name);
     [CallWith=ScriptState] Promise<boolean> unsubscribe();
 
     serializer;
 };