feat(browser): Add browser.isConnected() (#4403)

Add `browser.isConnected()` to the public api to be able to tell when the browser is connected

fixes https://github.com/smooth-code/jest-puppeteer/pull/237#issuecomment-490260041

experimental/puppeteer-firefox/lib/Browser.js

@@ -57,6 +57,13 @@ class Browser extends EventEmitter {
     this._connection.dispose();
   }
 
+  /**
+   * @return {boolean}
+   */
+  isConnected() {
+    return !this._connection._closed;
+  }
+
   /**
    * @return {!BrowserContext}
    */