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This is in preparation for refreshing public account policy, which
doesn't have a PrefService at its disposal. CloudPolicyManager now takes
care of extracting the pref value and passing it on to
CloudPolicyRefreshScheduler.

BUG=chromium:152937
TEST=unit tests


Review URL: https://chromiumcodereview.appspot.com/11299305

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@170974 0039d316-1c4b-4281-b951-d872f2087c98