* FirebaseMessaging: `NSUserDefaults` replaced by `GULUserDefaults`
* FirebaseMessaging: `GoogleUtilities/UserDefaults` added to dependencies. `GoogleUtilities` dependencies version updated to `~> 5.3`
* FIRMessagingPubSub: imports order
Added functionality not exposed in public header yet. I would leave to next FCM SDK owner to expose this functionality (and follow any required review process).
Bugs:
72701086