Decouple update_client::NetworkFetcher interface from Chromium network.

This change is mechanical.

It enables NetworkFetcherFactory, which is injected by the embedder, and
which creates instances of NetworkFetcher.

Bug: 929167
Change-Id: I0246974ab18697dbfda49031666f04811e7b1bcf
Reviewed-on: https://chromium-review.googlesource.com/c/1476323
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633755}
14 files changed