All DCP manager tests that enroll/register are tested with all auths.
Use a FakeSigningService in the DeviceCloudPolicyInitializer in order
to be able to unit test re-enrollment for locked devices when
registering with a certificate.
BUG=
624187,
641447
TEST=unit_tests
Committed:
https://crrev.com/f71058accb8d57221145dfb2c34769244d6cc54d
Cr-Commit-Position: refs/heads/master@{#415574}