Remove OnCertDBChanged |cert| parameter.

It would be null in most cases, so no consumers bothered to try to use it.

BUG=none

Review-Url: https://codereview.chromium.org/2691683003
Cr-Commit-Position: refs/heads/master@{#451425}
diff --git a/chromeos/cert_loader_unittest.cc b/chromeos/cert_loader_unittest.cc
index d2ccb40..c42722b6 100644
--- a/chromeos/cert_loader_unittest.cc
+++ b/chromeos/cert_loader_unittest.cc
@@ -46,7 +46,7 @@
   ~TestNSSCertDatabase() override {}
 
   void NotifyOfCertAdded(const net::X509Certificate* cert) {
-    NSSCertDatabaseChromeOS::NotifyObserversCertDBChanged(nullptr);
+    NSSCertDatabaseChromeOS::NotifyObserversCertDBChanged();
   }
 };