Document the use of OS labels on security bugs.

Bug: None
Change-Id: I6a91e5cdeb9bd9a585c782eea1a5bf767f44147f
Reviewed-on: https://chromium-review.googlesource.com/782705
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518335}
2 files changed