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When sync receives DISABLED_BY_ADMIN from server PSS stops SyncBackendHost and 
disables sync by settings DisabledByAdmin pref. This prevents sync from 
connecting to server in the future. The way to reenable sync is to sign out and 
sign in again.
In future milestone we will implement retry logic that will periodically 
(once a day) try to connect to sync server to see if sync is still disabled.

BUG=178417

Review URL: https://chromiumcodereview.appspot.com/14655009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202956 0039d316-1c4b-4281-b951-d872f2087c98