initForPlayback(variants, offlineSessionIds, isLive = true) {
    this.storedPersistentSessions_ = new Map();

    for (const sessionId of offlineSessionIds) {
      this.storedPersistentSessions_.set(
          sessionId, {initData: null, initDataType: null});
    }

    for (const metadata of this.config_.persistentSessionsMetadata) {
      this.storedPersistentSessions_.set(
          metadata.sessionId,
          {initData: metadata.initData, initDataType: metadata.initDataType});
    }

    this.usePersistentLicenses_ = this.storedPersistentSessions_.size > 0;

    return this.init_(variants, isLive);
  }
Initialize the drm engine for playback operations.

@param {!Array<shaka.extern.Variant>} variants
   The variants that we want to support playing.
@param {!Array<string>} offlineSessionIds
@param {boolean=} isLive
@return {!Promise}