[ServiceWorker] openWindow with a WebApp manifests (#720)

When using a [web application manifest](http://w3c.github.io/manifest/) together with a service worker, this can happen:

1. The user opens a web application and installs it as application (using the manifest), for example in Chrome for Android's "Add to Homescreen".
2. The user uses the app as if it were a native application, for example when using "display"="standalone" in the application manifest.
3. The application gets a push notification. When clicking the notification, this code is executed (from [MDN](https://developer.mozilla.org/en-US/docs/Web/API/WindowClient/focus):
    ```javascript
    self.addEventListener('notificationclick', function(event) {
      console.log('On notification click: ', event.notification.tag);
      event.notification.close();

      // This looks to see if the current is already open and
      // focuses if it is
      event.waitUntil(clients.matchAll({
        type: "window"
      }).then(function(clientList) {
        for (var i = 0; i < clientList.length; i++) {
          var client = clientList[i];
          if (client.url == '/' && 'focus' in client)
            return client.focus();
        }
        if (clients.openWindow)
          return clients.openWindow('/');
      }));
    });
    ```
4. If only the web application is open, that is opened. If there is no client, a new tab/window will be opened in the browser, not the application saved to the homescreen as expected.

Maybe this is an implementation issue, but I think there should be a way for a service worker to specify how the window should be opened, or it should always be opened as an app if it is installed.

---
Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/720