Service worker installation uses an algorithm (Start Register) that doesn't exist in SW spec

[mgiuca]

Invoke Start Register with scope and src members of the registration, a new promise, client, manifest URL, plus the type and update_via_cache members of the registration, in which case the state of the settled promise determines whether the installation succeeded or not.

There is no "Start Register" in the SW spec. As far as I can tell, the nearest thing would be to simply invoke the register() method.

I'm also not sure why this method would (ever) have taken the manifest URL. If we can avoid passing the manifest URL, then we don't need to store it in this algorithm.

Also note that this feature doesn't seem to have been implemented by any user agent, so it is considered "at risk" (though I still believe it would be useful which is why I haven't pushed to deleted it yet).