code: function createExecutor(url) {
    let agent = new http.Agent({ keepAlive: true });
    let client = url.then(url => new http.HttpClient(url, agent));
    let executor = new http.Executor(client);
    configureExecutor(executor);
    return executor;
  }
docs: Creates a command executor with support for ChromeDriver's custom commands.
  @param {!Promise<string>} url The server's URL.
  @return {!command.Executor} The new command executor.