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This creates the DlcserviceClient to call into dlcservice daemon's exposed
DBus API. The dlcservice daemon provides management of DLC
(Downloadable Content) modules. Exposures from DlcserviceClient is to
allow for Install/Uninstall/GetInstalled in a manner that wraps the
respective DBus API exported from the dlcservice daemon in a Chrome
style callable interface.

Information on building a DLC and how what dlcservice daemon is can be
followed at:
https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice

Bug: 1000711
Test: autoninja -C out/Default chromeos_unittests && testing/xvfb.py out/Default/chromeos_unittests --gtest_filter=DlcserviceClientTest*
Test: autoninja -C out/Default unit_tests && testing/xvfb.py ./out/Default/unit_tests
Test: autoninja -C out_${SDK_BOARD}/Release chrome nacl_helper && deploy_chrome --build-dir=out_${SDK_BOARD}/Release --to=$TEST_IP
Test: modifications to actually call into to DlcserviceClient with nebraska.py as payload path server. $> restart ui
Test: Custom CHROMEOS_AUSERVER within /mnt/stateful_partition/etc/lsb-release on DUT with local nebraska.py port.
Change-Id: Ia269cab18d7fcc4c1c6d0ee72fb3383b11af38a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1802682
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697691}