GitLab

Adds PrefMetricsService methods to track changes to user preferences
that happen outside of Chrome and report discrepancies via UMA.

BUG=266569

Review URL: https://chromiumcodereview.appspot.com/22676002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221944 0039d316-1c4b-4281-b951-d872f2087c98