code: function(auditRuleName, severity) {
          if (!(auditRuleName in this.rules_))
              this.rules_[auditRuleName] = {};
          this.rules_[auditRuleName].severity = severity;
      }
docs: Sets the user-specified severity for the given audit rule. This will
  replace the default severity for that audit rule in the audit results.
  @param {string} auditRuleName
  @param {axs.constants.Severity} severity