static indexUsedRules(rules, indexedStylesheets) {
    rules.forEach(rule => {
      const stylesheetInfo = indexedStylesheets[rule.styleSheetId];

      if (!stylesheetInfo) {
        return;
      }

      if (rule.used) {
        stylesheetInfo.usedRules.push(rule);
      }
    });
  }
Adds used rules to their corresponding stylesheet.
@param {Array<LH.Crdp.CSS.RuleUsage>} rules The output of the CSSUsage gatherer.
@param {Object<string, StyleSheetInfo>} indexedStylesheets Stylesheet information indexed by id.