Add msarda@ as owner for most of the signin code.

This CL adds msarda@ as owner for the most of the sign-in code as his team
owns now the Chrome sign-in feature in Chromium.

BUG=NONE

Review-Url: https://codereview.chromium.org/2583533002
Cr-Commit-Position: refs/heads/master@{#439070}

chrome/browser/signin/OWNERS

-atwilson@chromium.org
 mlerman@chromium.org
+msarda@chromium.org
 rogerta@chromium.org
 
 per-file chrome_proximity_auth_*=tbarzic@chromium.org

chrome/browser/ui/sync/OWNERS

 maxbogue@chromium.org
 pavely@chromium.org
+msarda@chromium.org
 rogerta@chromium.org
 stanisc@chromium.org
 zea@chromium.org

components/signin/OWNERS

-atwilson@chromium.org
+msarda@chromium.org
 rogerta@chromium.org

google_apis/gaia/OWNERS

+msarda@chromium.org
 rogerta@chromium.org
 zelidrag@chromium.org