Add a histogram for measuring the number of times we fall back to common name matching, when a cert…

net/cert/cert_verify_proc_mac.cc

Index: net/cert/cert_verify_proc_mac.cc
diff --git a/net/cert/cert_verify_proc_mac.cc b/net/cert/cert_verify_proc_mac.cc
index 4efdaacffed3830465ae0f5d96915bed5dd8d5de..542fba1151459814e7629aa12f23fa1ad3022494 100644
--- a/net/cert/cert_verify_proc_mac.cc
+++ b/net/cert/cert_verify_proc_mac.cc
@@ -656,8 +656,10 @@ int CertVerifyProcMac::VerifyInternal(
   // Perform hostname verification independent of SecTrustEvaluate. In order to
   // do so, mask off any reported name errors first.
   verify_result->cert_status &= ~CERT_STATUS_COMMON_NAME_INVALID;
-  if (!cert->VerifyNameMatch(hostname))
+  if (!cert->VerifyNameMatch(hostname,
+                             &verify_result->common_name_fallback_used)) {
     verify_result->cert_status |= CERT_STATUS_COMMON_NAME_INVALID;
+  }
 
   // TODO(wtc): Suppress CERT_STATUS_NO_REVOCATION_MECHANISM for now to be
   // compatible with Windows, which in turn implements this behavior to be