async _getSitemapXml(sitemapUrl, options, depth, previousUrl) {
    let sitemapXml = await this._cache.get(sitemapUrl);
    if (!sitemapXml) {
      try {
        sitemapXml = await rp(sitemapUrl);
      } catch (error) {
        extend(error, { options, depth, previousUrl });
        this.emit(HCCrawler.Events.SitemapXmlRequestFailed, error);
        sitemapXml = EMPTY_TXT;
      } finally {
        await this._cache.set(sitemapUrl, '1');
      }
    }
    return sitemapXml;
  }
@param {!string} sitemapUrl
@param {!Object} options
@param {!number} depth
@param {string} previousUrl
@return {!Promise<!string>}