1. Start subscription.
2. Unregister Service Worker.
3. Subscription finishes successfully, which calls PersistToPrefs from PushMessagingServiceImpl::DidSubscribeWithEncryptionInfo.

Then we'll reach PushMessagingMessageFilter::PersistRegistrationOnIO, which will fail to store to the Service Worker DB, and hence reject the JS subscribe() promise, but it never cleans up the PushMessagingAppIdentifier that we stored in #3.

We should clear that up sometime...