cleanContinuityCache_(periods) {
    const activePeriodId = new Set(periods.map((p) => p.id));
    for (const key of this.continuityCache_.keys()) {
      if (!activePeriodId.has(key)) {
        this.continuityCache_.delete(key);
      }
    }
  }
Clean continuityCache Object to remove reference of removed periods.
This should end up running after the current manifest has been processed
so that it can use previous periods to calculate the continuity of the new
periods.
@param {!Array<shaka.extern.Period>} periods
@private