Add headless function for Firefox Options (#37752)

types/selenium-webdriver/firefox.d.ts

@@ -41,6 +41,12 @@ export class Options extends webdriver.Capabilities {
    * @see https://github.com/mozilla/geckodriver
    */
   useGeckoDriver(enable: boolean): Options;
+
+  /**
+   * Configures the geckodriver to start Firefox in headless mode.
+   * @return {!Options} A self reference.
+   */
+  headless(): Options
 }
 
 /**

types/selenium-webdriver/test/firefox.ts

@@ -18,6 +18,7 @@ function TestFirefoxOptions() {
     options = options.setBinary('binary');
     options = options.setProfile('profile');
     options = options.setProxy({ proxyType: 'proxy' });
+    options = options.headless();
 }
 
 function TestServiceBuilder() {