[Sync] Rely on directory to decide which types need to be purged
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