Add mastiz@ as owner of sync

As per ownership handover, and now that I feel comfortable enough with
the codebase.

Bug: None
Change-Id: Ib96afc8e4d8a45681f8124b68758c10646b6da71
Reviewed-on: https://chromium-review.googlesource.com/940137Reviewed-by: Nicolas Zea (slow) <zea@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540113}

components/sync/OWNERS

+mastiz@chromium.org
 pavely@chromium.org
 skym@chromium.org
 zea@chromium.org