constructor() {
    /**
     * @type {string}
     */
    this.url;

    /**
     * @type {function(?, string=)}
     */
    this.callback;

    /**
     * @type {string}
     */
    this.method;

    /**
     * @type {?string}
     */
    this.content;

    /**
     * @type {!Object<string, string>}
     */
    this.headers;
  }
A data model for a network request.
@record @struct
@private