[login_manager] Stop Guest login from taking ownership

I introduced a regression when I moved owner key generation to the session manager.
Fix it by ensuring that we don't start key generation for the 'incognito' user.

BUG=12574
STATUS=Fixed
TEST=unit tests, login_OwnershipTaken

Change-Id: I1ab6dce50eda41523835f473cb3294caba704690

Review URL: http://codereview.chromium.org/6598076
1 file changed