title: getSubscription returns a null subscription
body: <p>I am new to service workers and GAE, I am able to register the service workers but not able to subscribe the PushManager, getting subscription null error. 
  Find the below code for reference.</p>
  
  <pre><code>serviceWorkerRegistration.pushManager.getSubscription()  
    .then(function(subscription) {  
      var pushButton = document.querySelector('.js-push-button');  
      pushButton.disabled = false;
  
      if (!subscription) {  
        console.log('subscription error '); 
        return;  
      }
      console.log('subscriptioned ');
  
      // Keep your server in sync with the latest subscriptionId
      sendSubscriptionToServer(subscription);
  
      // Set your UI to show they have subscribed for  
      // push messages  
      pushButton.textContent = 'Disable Push Messages';  
      isPushEnabled = true;  
    })  
    .catch(function(err) {  
      console.warn('Error during getSubscription()', err);  
    });
  });
  </code></pre>
  
  <p>In above code getting "subscription" value as "null" inside then, so that, control is coming to if block and simply returning.</p>