Expect |callback| to be non-null in PolicyCertVerifier.

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}

chrome/browser/chromeos/policy/policy_cert_verifier.cc

@@ -36,7 +36,6 @@ void CompleteAndSignalAnchorUse(const base::Closure& anchor_used_callback,
                                 int error) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
   MaybeSignalAnchorUse(error, anchor_used_callback, *verify_result);
-  if (completion_callback)
   std::move(completion_callback).Run(error);
 }
 
@@ -77,6 +76,7 @@ int PolicyCertVerifier::Verify(const RequestParams& params,
                                const net::NetLogWithSource& net_log) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
   DCHECK(delegate_);
+  DCHECK(completion_callback);
   net::CompletionOnceCallback wrapped_callback =
       base::BindOnce(&CompleteAndSignalAnchorUse, anchor_used_callback_,
                      std::move(completion_callback), verify_result);

chrome/browser/chromeos/policy/policy_cert_verifier.h

@@ -44,7 +44,6 @@ class PolicyCertVerifier : public net::CertVerifier {
   void SetTrustAnchors(const net::CertificateList& trust_anchors);
 
   // CertVerifier:
-  // Note: |callback| can be null.
   int Verify(const RequestParams& params,
              net::CRLSet* crl_set,
              net::CertVerifyResult* verify_result,