pollExpiration_() {
    this.activeSessions_.forEach((metadata, session) => {
      const oldTime = metadata.oldExpiration;
      let newTime = session.expiration;
      if (isNaN(newTime)) {
        newTime = Infinity;
      }

      if (newTime != oldTime) {
        this.playerInterface_.onExpirationUpdated(session.sessionId, newTime);
        metadata.oldExpiration = newTime;
      }
    });
  }
Called in an interval timer to poll the expiration times of the sessions.
We don't get an event from EME when the expiration updates, so we poll it
so we can fire an event when it happens.
@private