GitLab

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
TBR=pastarmovj

Review-Url: https://codereview.chromium.org/2303483002
Cr-Commit-Position: refs/heads/master@{#415799}