Don't verify the policy timestamp when loading from cache.

There are conditions that cause a device's clock to go back in time, and then
if the policy is rejected because its timestamp is in the future then no user
can sign-in, and guest sessions can't be started.

This CL removes the timestamp check from policies loaded from the cache;
new policies downloaded from the server or downloaded during enrollment still
have their timestamps checked.

BUG=313906,265507,303508
R=bartfab@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/62453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233323 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233323