libchromeos: add GetKeys() method to KeyValueStore

GetKeys returns the keys currently set in the key value store.

BUG=none
TEST=ran unit tests

Change-Id: Ifd1c1f7678df6fd9b68900e2573d5e71802aa004
Reviewed-on: https://chromium-review.googlesource.com/253430
Trybot-Ready: Aaron Kemp <kemp@google.com>
Tested-by: Aaron Kemp <kemp@google.com>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Aaron Kemp <kemp@google.com>
(cherry picked from commit 4b93f8e27b4d611a0fa106716527231b1fe73db5)
Reviewed-on: https://chromium-review.googlesource.com/255785
Reviewed-by: Aaron Kemp <kemp@google.com>
Commit-Queue: Patrick Sosinski <sosinski@google.com>
Tested-by: Patrick Sosinski <sosinski@google.com>
3 files changed