title: firebase cloud messaging: setBackgroundMessageHandler not called
body: <p>I am prototyping browser push notifications with FCM. I just copied the example code from the quickstart (<a href="https://github.com/firebase/quickstart-js/tree/master/messaging" rel="noreferrer">https://github.com/firebase/quickstart-js/tree/master/messaging</a>). Messages are recieved and displayed as they should be. But when I try to modify the message in the Service Worker (messaging.setBackgroundMessageHandler) nothing happens. The service worker is called, and if I implement an event listener in that service worker for the push notifications, it catches the event. But the method setBackgroundMessageHandler is never called. 
  I am trying this on Chrome 54.</p>
  
  <p>Any ideas what I need to do to customize the message in the service worker?</p>
  
  <p>Thank you very much!</p>