Add asvitkine to chrome/browser/metrics/OWNERS.

I'm now very familiar with the MetricsService code
and believe it makes sense for me to become an
OWNER here.

BUG=none

Review URL: https://codereview.chromium.org/191003004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255966 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/metrics/OWNERS

 jar@chromium.org
 rtenneti@chromium.org
 isherman@chromium.org
+asvitkine@chromium.org