function buildCommonMessage() {
  return {
    'message': {
      'topic': 'news',
      'notification': {
        'title': 'FCM Notification',
        'body': 'Notification from FCM'
      }
    }
  };
}
Construct a JSON object that will be used to define the
common parts of a notification message that will be sent
to any app instance subscribed to the news topic.