code: function onMessageReceivedSubscriptionState() {
      let retrievedPushSubscription = null;
      self.registration.pushManager.getSubscription()
          .then(pushSubscription => {
              retrievedPushSubscription = pushSubscription;
              if (!pushSubscription) {
                  return null;
              } else {
                  return self.registration.pushManager.permissionState(
                      pushSubscription.options
                  );
              }
          }).then(permissionStateOrNull => {
              if (permissionStateOrNull == null) {
                  broadcastReply(WorkerMessengerCommand.AMP_SUBSCRIPTION_STATE, false);
              } else {
                  const isSubscribed = !!retrievedPushSubscription &&
                      permissionStateOrNull === 'granted';
                  broadcastReply(WorkerMessengerCommand.AMP_SUBSCRIPTION_STATE,
                      isSubscribed);
              }
          });
  }
docs: Broadcasts a single boolean describing whether the user is subscribed.