* **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 title_;
}
base::string16 Notification::GetSubtitle() const {
  return subtitle_;
}
base::string16 Notification::GetBody() const {
  return body_;
}
bool Notification::GetSilent() const {
  return silent_;
}
bool Notification::GetHasReply() const {
  return has_reply_;
}
base::string16 Notification::GetTimeoutType() const {
  return timeout_type_;
}
base::string16 Notification::GetReplyPlaceholder() const {
  return reply_placeholder_;
}
base::string16 Notification::GetSound() const {
  return sound_;
}
base::string16 Notification::GetUrgency() const {
  return urgency_;
}
std::vector<electron::NotificationAction> Notification::GetActions() const {
  return actions_;
}
base::string16 Notification::GetCloseButtonText() const {