function getStylesheetText(url, clients) {
        return _send(
            {
                method: "CSS.getStylesheetText",
                params: {
                    url: url
                }
            },
            clients
        );
    }
Protocol method. Rretrieves the content of a given stylesheet (for unit testing)
@param {number|Array.<number>} clients A client ID or array of client IDs that should navigate to the given URL.
@param {string} url Absolute URL that identifies the stylesheet.
@return {$.Promise} A promise that's resolved with the return value from the first client that responds
     to the method.