More OWNERS for docs/security.

BUG=NONE

Change-Id: I6976ea94e9fb6fef2a300d3c70209dc36e289e0a
Reviewed-on: https://chromium-review.googlesource.com/594887Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490797}

docs/security/OWNERS

+awhalley@chromium.org
 elawrence@chromium.org
 estark@chromium.org
 felt@chromium.org
@@ -7,3 +8,4 @@ palmer@chromium.org
 parisa@chromium.org
 rsesek@chromium.org
 tsepez@chromium.org
+vakh@chromium.org