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This CL starts the System-proxy worker which tunnels ARC++ traffic
according to the |ArcEnabled| policy and forwards the address of the
local proxy to ARC++ through the |ArcSettingsService|.
It also introduces a new user preference that keeps track of the
address of the local proxy worker and syncs with ARC when the address
changes or System-proxy is disabled by policy.

Bug: 1042639
Test: unit tests, browser test, manual test on DUT
Change-Id: Ibe84d74353d5f75376393e7cd8745f63db073de7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2323521Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: David Jacobo <djacobo@chromium.org>
Reviewed-by: Omar Morsi <omorsi@google.com>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>
Cr-Commit-Position: refs/heads/master@{#795459}