title: Is it possible to receive Firebase Silent Push Notification?
body: <p>I have been developing Firebase Push Notification Integrated iOS application.
  I already integrated some codes, and some settings.
  So I can receive Firebase Silent Push Notification when app is in foreground.
  But I have a issue when app is closed.
  I close app. And I send Push Notification to closed app on other iPhone.
  Closed app would not receive Push Notification. But if I open closed app later, I want to know that there are a few missing Push Notifications while app has been closed.
  Is it possible to  know it ? 
  And Could I handle missing Push Notifications?</p>