network service: pass a URLLoaderFactory to renderers on SW registration

network service: pass a URLLoaderFactory to renderers on SW registration

This hooks up a URLLoaderFactory to be used by renderers that register a
service worker. Currently, it's implemented by the browser, and in turn
simply forwards on to the real network service.

This is "2." of https://bugs.chromium.org/p/chromium/issues/detail?id=715640#c3 .

BUG=715640