GitLab

"GetExistingDlcs" DBus method constant is in system_api now and can be
used within dlcservice client files.

BUG=none
TEST=./unit_tests

Change-Id: I347fbabef4691f69ec3482ffbf51cbe7c9b54c59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2199679
Auto-Submit: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768511}