Move device_management_backend.proto closer to the server version.

No functional change, just updating comments and moving a block around.

BUG=645078

Review-Url: https://codereview.chromium.org/2320873004
Cr-Original-Commit-Position: refs/heads/master@{#421213}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9b72e27717e3713ae3ca673f0afbfd99aa951c28
1 file changed