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For users of DlcserviceClient, the possibliy to observe the
download/install of a DLC is added.

Bug: b:143098235
Test: autoninja -C out/Default chromeos_unittests && ./out/Default/chromeos_unittests
Change-Id: I846f05664e1ff90626ac79af4dcc4e3dbfd7ce23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874815
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708828}