GitLab

Adds a real and a fake implementation of a D-Bus client that talks to
the the System-proxy daemon in Chrome OS.

BUG=chromium:1042621
TEST=unit_tests  --gtest_filter=*SystemProxySettingsPolicyHandlerTest*
     manually tested on a device

Change-Id: I456be37c7e540ec30afa2e4d5d5832e33112867e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2051847Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>
Cr-Commit-Position: refs/heads/master@{#744212}