title: Issue with .subscribe() method of pushManager
body: <p>I am following this very clear <a href="https://developers.google.com/web/fundamentals/push-notifications/subscribing-a-user#subscribe_a_user_with_pushmanager" rel="nofollow noreferrer">tutorial</a>.</p>
  
  <p>All works except that when I get to the stage <strong>subscribing a user with pushManager</strong> when I call <code>pushManager.subscribe()</code> I don't get any response from the promise whose status is <code>pending</code>.</p>
  
  <p>Maybe I am missing some glaring problem with my setup, which is as follows:</p>
  
  <pre><code>    if ('serviceWorker' in navigator &amp;&amp; 'PushManager' in window) {
        console.log('Service Worker and Push is supported');
  
        navigator.serviceWorker.register('service-worker.js')
        .then(function(swReg) {
          console.log('Service worker successfully registered.');
  
          navigator.serviceWorker.ready.then(function() {
  
            const subscribeOptions = {
              userVisibleOnly: true,
              applicationServerKey: ... // My application server key.
            };
  
            let promise = swReg.pushManager.subscribe(subscribeOptions)
            console.log(promise) // Promise {[[PromiseStatus]]: "pending", [[PromiseValue]]: undefined}
            promise
            .then(function(pushSubscription) {
  
              console.log('Received PushSubscription: ', pushSubscription); // Never gets called.
            })
            .catch(function(err) {
              console.error('Unable to subscribe.', err); // Never gets called.
            });
          })
        })
        .catch(function(err) {
          console.error('Unable to register service worker.', err);
        });
      }
  </code></pre>
  
  <p>Should additional configuration be made somewhere else? Any advice is appreciated.</p>