Enabling service workers when opener is not a secure context

[annevk]

From w3c/ServiceWorker#890 (comment):

Refining the above, seems like we should auto-disown opener for service worker controlled pages if opener is a non-secure context.

The current text that tries to accomplish this is broken as discovered in #2671.

@mikewest what exactly is the story of secure contexts and auxiliary browsing contexts?

[annevk]

cc @jungkees @jakearchibald