Update PushEvent and PushMessageData to match the spec.

Source/modules/push_messaging/PushEventInit.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
 
+// TODO(peter): Use BufferSource when union types can refer to other union types.
+typedef (ArrayBuffer or ArrayBufferView or USVString) PushMessageDataInit;
+
 dictionary PushEventInit : ExtendableEventInit {
-    PushMessageData data;
+    PushMessageDataInit data;
 };