DeviceManagementBackend: clarify field comment.

Add description of the timestamp.

Bug: 1006899
Change-Id: I80a94ce923ffc7a0d7a462dbd91a6dddc97f9b38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1933736
Commit-Queue: Alex Vallee <avallee@chromium.org>
Auto-Submit: Alex Vallee <avallee@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718680}
1 file changed