code: function onMessageReceivedUnsubscribe() {
      self.registration.pushManager.getSubscription()
          .then(subscription => subscription.unsubscribe())
          .then(() => {
              clearLocalDatabase();
              // OPTIONALLY IMPLEMENT: Forward the unsubscription to your server here
              broadcastReply(WorkerMessengerCommand.AMP_UNSUBSCRIBE, null);
          });
  }
docs: Unsubscribes the subscriber from push.
  The broadcast value is null (not used in the AMP page).