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According to
https://cs.chromium.org/chromium/src/net/cert/cert_verifier.h?l=143,
CertVerifier::Verify() must not be called with a null |callback|.
This CL modifies expectation and comment accordingly in PolicyCertVerifier.

This CL is a follow-up to the reviewer's comment at
https://crrev.com/c/1117917/6/chrome/browser/chromeos/policy/policy_cert_verifier.h#47.

Change-Id: If45d08a35c2d2add9e7e111b7cb69810c07788fe
Reviewed-on: https://chromium-review.googlesource.com/1127920Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573836}