def results_directory(self):
        """Returns the absolute path directory which will store all web tests outputted
        files. It may include a sub directory for artifacts and it may store performance test results."""
        if not self._results_directory:
            option_val = self.get_option(
                'results_directory') or self.default_results_directory()
            assert not self._filesystem.basename(option_val) == 'layout-test-results', (
                'crbug.com/1026494, crbug.com/1027708: The layout-test-results sub directory should '
                'not be passed as part of the --results-directory command line argument.')
            self._results_directory = self._filesystem.abspath(option_val)
        return self._results_directory