_addGatherStep(gatherResult, flags) {
    const gatherStep = {
      artifacts: gatherResult.artifacts,
      flags,
    };
    this._gatherSteps.push(gatherStep);
    this._gatherStepRunnerOptions.set(gatherStep, gatherResult.runnerOptions);
  }
@param {LH.Gatherer.GatherResult} gatherResult
@param {LH.UserFlow.StepFlags} [flags]