Separate push event data from the rest of the W3C Push API flag.

Source/modules/push_messaging/PushEvent.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/#idl-def-PushEvent
 
 [
     Constructor(DOMString type, optional PushEventInit eventInitDict),
     Exposed=ServiceWorker,
     RuntimeEnabled=PushMessaging,
 ] interface PushEvent : ExtendableEvent {
-    readonly attribute PushMessageData data;
+    [RuntimeEnabled=PushMessagingData] readonly attribute PushMessageData data;
 };