add more log to quit

src/puppet-web/browser.js

@@ -168,11 +168,11 @@ class Browser extends EventEmitter {
       log.silly('PuppetWebBrowser', 'quit() co()')
       yield this.driver.close() // http://stackoverflow.com/a/32341885/1123955
       yield this.driver.quit()
+      log.silly('PuppetWebBrowser', 'quit() this.driver = null')
       this.driver = null
 
       yield this.clean()
       log.silly('PuppetWebBrowser', 'quit() co() end')
-
     }).catch(e => {
       // console.log(e)
       // log.warn('PuppetWebBrowser', 'err: %s %s %s %s', e.code, e.errno, e.syscall, e.message)