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Preivously we relied on the SyncBackendRegistrar's last configured types to
decide which types have been recently disabled. Now we just purge all disabled
types that exist in the directory, as recorded by their progress markers (which
should never be deleted unless the type has been purged).

BUG=386778

Review URL: https://codereview.chromium.org/398813005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284283 0039d316-1c4b-4281-b951-d872f2087c98