Adding OWNERS in /third_party/libsync

Making sure there are at least 2 owners after I leave.
This CL was uploaded by git cl split.

R=dnicoara@chromium.org

Change-Id: Ib08fe6f8c354dbd7f9dc6c3c91caa693a493579a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674611
Auto-Submit: Antoine Labour <piman@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671892}

third_party/libsync/OWNERS

-reveman@chromium.org
+dcastagna@chromium.org
+dnicoara@chromium.org
 piman@chromium.org