Split the policy refresh rate preference into user- and device-policy refresh rate.

This is an effort towards removing dependencies of the ProfilePolicyConnector on Profile.

BUG=none
TEST=Verify that user- and device-refresh rate are set correctly when specified by policy.


Review URL: http://codereview.chromium.org/7014036

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88003 0039d316-1c4b-4281-b951-d872f2087c98