async function getBenchmarkIndex(executionContext) {
  const status = {msg: 'Benchmarking machine', id: 'lh:gather:getBenchmarkIndex'};
  log.time(status);
  const indexVal = await executionContext.evaluate(pageFunctions.computeBenchmarkIndex, {
    args: [],
  });
  log.timeEnd(status);
  return indexVal;
}
Computes the benchmark index to get a rough estimate of device class.
@param {LH.Gatherer.Driver['executionContext']} executionContext
@return {Promise<number>}