Property indicating the time at which, if any, the subscription won't be valid anymore. The user agent will attempt to fire a `pushsubscriptionchange` event with a new subscription before this happens. Chrome will always return NULL, until we support subscription refreshes.
Docs: https://developer.mozilla.org/en-US/docs/Web/API/PushSubscription https://developer.mozilla.org/en-US/docs/Web/API/PushSubscription/expirationTime