Make Service Worker registration work properly (don't use outdated steps) - #792

@marcoscaceres While I'm on a roll with Manifest...

Edit: Please look at this delta.

1. I need a bit of help with ReSpec. (General ReSpec comments appreciated; I've been trying out the new (?) |var|.{{Method}} syntax.) I tried to add a reference to RegistrationOptions.type using "for="RegistrationOptions"" (the "for" clause working well for all the other methods and members). But I get a conflict with the other definition of ECMAScript's Type() function. I thought "for" was supposed to disambiguate it by saying "this type is for RegistrationOptions only" but it still conflicts.
2. This is a bit nasty but it's in the current text too: (the old text reads: "the state of the settled promise determines whether the installation succeeded or not") --- the fact that if service worker registration fails, the whole installation is aborted, even though service worker registration is optional. So some user agents will not bother to register a (failing) SW, thus successfully installing, and others will try to register the SW and fail the install. Is that "okay" (essentially making a best effort to check, but you don't have to check). Or should we succeed the install even if SW registration fails, for consistency across user agents?

The upstream PR has now merged, so this PR's delta is correct.

Well Jeffrey (talked at TPAC) says to please not do that as he is going to use that innWeb Packaging which uses web app manifest

@jyasskin could you comment?

Sure, but the multi-implementer problem remains. Not sure at this point we want to support this in Gecko, but need to check if WebKit supports it.

We might use the serviceworker field to have navigation to a web package automatically install its service worker before the start page loads, but I think we can re-add the field if/when we do that. It won't hurt us to remove it until then.

Ok, let's proceed with the plan to remove it... we can always bring it back.

Oops, thanks for closing this out.