Start device policy core if policy is present.

Previously, we'd only start if the policy indicated that the client is
actively managed. That's wrong because a device in state UNMANAGED
needs to be able to fetch policy to get out of that state again. Also
add a new unit test to cover this scenario.

BUG=chromium:329203

Review URL: https://codereview.chromium.org/98143009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241556 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241556