Use ASCII serialization for origins

[annevk]

In whatwg/html#2568 we found out we need to remove the Unicode serialization feature. Once the HTML Standard is changed it would be good for Service Workers to align with that change and to also update any corresponding tests (or write tests).

Once the HTML Standard is changed you should be able to use "serialization of an origin" as the new linking text in Bikeshed. Probably after a day once Shepherd has picked it up.