static async compute_(data, context) {
    const {CSSUsage, Stylesheets, devtoolsLog} = data;
    const networkRecords = await NetworkRecords.request(devtoolsLog, context);
    const indexedSheets = UnusedCSS.indexStylesheetsById(Stylesheets, networkRecords);
    UnusedCSS.indexUsedRules(CSSUsage, indexedSheets);

    const items = Object.keys(indexedSheets)
      .map(sheetId => UnusedCSS.mapSheetToResult(indexedSheets[sheetId]));
    return items;
  }
@param {{Stylesheets: LH.Artifacts['Stylesheets'], CSSUsage: LH.Artifacts['CSSUsage'], devtoolsLog: LH.DevtoolsLog}} data
@param {LH.Artifacts.ComputedContext} context
@return {Promise<LH.Audit.ByteEfficiencyItem[]>}