Update throttling/cache/js state on the SW target

In [Developer Diary #Day1 - YouTube](https://www.youtube.com/watch?annotation_id=annotation_3128105329&feature=iv&src_vid=L50YZrpcEqA&v=CubrYp4cuyY) @paullewis ran into a weird behavior that the Service Worker wasnt going offline, despite Lighthouse making the page offline. It was filed as [crbug 648208](https://bugs.chromium.org/p/chromium/issues/detail?id=648208), and duped into a WontFix, as the devtools frontend handles this case decently well. The frontend will connect to both targets and send the network throttling commands to each.

In our current behavior ...
- sw network requests are unaffected by the throttling
- sw doesn't go offline when the page goes offline.

Currently Lighthouse only sends the throttling stuff to the page's target. It'd make sense to also connect to the SW and flip it offline etc.