[ServiceWorker] Restrict openWindow() to http(s) schemes? (#699)

We currently reject on `about:blank` (due to #696), but is there any benefit in allowing schemes other than `http` and `https`?

What would it mean to `openWindow('mailto:hello@example.com')`? etc

I propose we reject on schemes other than http/https. Any objections?

---
Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/699