Also add comments explaining why the one that are still around are still
around.

Change-Id: Id8ce1fa3fe2a3bc97a179018737cc8d027b1068a
Reviewed-on: https://chromium-review.googlesource.com/250878Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>