static computeCacheLifetimeInSeconds(headers, cacheControl) {
    if (cacheControl && cacheControl['max-age'] !== undefined) {
      return cacheControl['max-age'];
    }

    const expiresHeaders = headers.get('expires');
    if (expiresHeaders) {
      const expires = new Date(expiresHeaders).getTime();
      // Invalid expires values MUST be treated as already expired
      if (!expires) return 0;
      return Math.ceil((expires - Date.now()) / 1000);
    }

    return null;
  }
Return max-age if defined, otherwise expires header if defined, and null if not.
@param {Map<string, string>} headers
@param {ReturnType<typeof parseCacheControl>} cacheControl
@return {?number}