network service, service worker: create different urlloader for context

network service, service worker: create different urlloader for context

This gives the context requests a different URLLoaderImpl. It it still
simply forwarding to the real network service. This is enough to get a
basic page
(https://googlechrome.github.io/samples/service-worker/basic/index.html)
up and running without DCHECK and have the service worker show up in dev
tools as registered.

The next step is probably to get the Controllee to actually request data
from the SWVersion.

BUG=715640