Change UserPolicySigninService to not create SigninManager in Observe()

Add unit test to SigninManager to test kSigninAllowed=false case.

Update UserPolicySigninService to no longer fetch a SigninManager in its
Observe() callback. Since SigninManager can generate a SIGNED_OUT notification
during initialization, trying to access SigninManager from the callback can
lead to a crash when we double-create SigninManager.

BUG=316229
R=joaodasilva@chromium.org

Review URL: https://codereview.chromium.org/66553003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234464 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234464