getSessionIds() {
    const sessions = this.activeSessions_.keys();
    const ids = shaka.util.Iterables.map(sessions, (s) => s.sessionId);

    // TODO: Make |getSessionIds| return |Iterable| instead of |Array|.
    return Array.from(ids);
  }
Returns the ID of the sessions currently active.

@return {!Array<string>}