* **Electron Version:**
  * 5.0.x
* **Operating System:**
  * macOS 10.14.5

### Expected Behavior
Electron does not crash at exit.

### Actual Behavior
The notifications are sent well, i use the close() function to avoid persistence in Notification Center. 

By doing this every time I quit the application I have a message that warns me that the application did not stop properly.

### To Reproduce
```javascript
const notif = new Notification({title: title, body: body, silent: true})
notif.on('click', () => appEvent.emit('show-window'))
notif.show()
setTimeout(() => notif.close(), 10000)
```
After sending notification(s), close application (app.quit()) and the error will appear.

To no longer have the error after quitting app, i do not use the function close () but my application generates a lot of notifications and I would have preferred to handle that.

shell/browser/api/atom_api_notification.cc
return close_button_text_;
}
void Notification::SetTitle(const base::string16& new_title) {
  title_ = new_title;
}
void Notification::SetSubtitle(const base::string16& new_subtitle) {
  subtitle_ = new_subtitle;
}
void Notification::SetBody(const base::string16& new_body) {
  body_ = new_body;
}
void Notification::SetSilent(bool new_silent) {
  silent_ = new_silent;
}
void Notification::SetHasReply(bool new_has_reply) {
  has_reply_ = new_has_reply;
}
void Notification::SetTimeoutType(const base::string16& new_timeout_type) {
  timeout_type_ = new_timeout_type;
}
void Notification::SetReplyPlaceholder(const base::string16& new_placeholder) {
  reply_placeholder_ = new_placeholder;
}
void Notification::SetSound(const base::string16& new_sound) {
  sound_ = new_sound;
}
void Notification::SetUrgency(const base::string16& new_urgency) {
  urgency_ = new_urgency;
}
void Notification::SetActions(