Fix service worker onUpdate not triggering if page load was interrupted

[TheRusskiy]

We need to handle the case when a page is loaded and a service worker is already "waiting".

To me this happens randomly when you least expect, but one way to reproduce this issue is to open the page,
see that service worker is beginning to download and close / reload the page.

It seems that Chrome finishes the SW load even if you close the page. In this case on the next page load it's waiting and not installing, which means the
registration.waiting.postMessage({ type: "SKIP_WAITING" }) trick won't work.