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This CL implements requesting proxy authentication credentials from the
NetworkService instance of the primary user Profile's default
StoragePartition when System-proxy signals that it requires credentials.

System-proxy is a new system service on CrOS that performs
authentication and connection setup for CrOS services and ARC apps
that are proxy aware, but can't perform the authentication challenge
themselves.

Bug:1042642
Test:unittests, services_unittests, manually on DUT

Change-Id: I1d79da46ee9537c75131ff5e734fb39e525e0d5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2279193
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>
Reviewed-by: Eric Roman <eroman@chromium.org>
Reviewed-by: Andreea-Elena Costinas <acostinas@google.com>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Omar Morsi <omorsi@google.com>
Cr-Commit-Position: refs/heads/master@{#790345}