Right now NotificationData is used by notification_service.mojom but not by service_worker_event_dispatcher.mojom[1], which still uses PlatformNotificationData as defined by IPC Enum traits in platform_notification_messages.h[2]. 

We should delete platform_notification_messages.h and use the NotificationData mojo struct & struct traits everywhere.

[1] https://cs.chromium.org/chromium/src/content/common/service_worker/service_worker_event_dispatcher.mojom

[2] https://cs.chromium.org/chromium/src/content/common/platform_notification_messages.h