We should use UserID object to identify users instead of username.

chromeos/attestation/mock_attestation_flow.h

Index: chromeos/attestation/mock_attestation_flow.h
diff --git a/chromeos/attestation/mock_attestation_flow.h b/chromeos/attestation/mock_attestation_flow.h
index efefd94f3125651e9c882a062016d5d858c95f96..e31d7575bd5e2b240b6ddd4a55d4195055a49b16 100644
--- a/chromeos/attestation/mock_attestation_flow.h
+++ b/chromeos/attestation/mock_attestation_flow.h
@@ -9,6 +9,7 @@
 
 #include "base/basictypes.h"
 #include "base/callback.h"
+#include "components/user_manager/user_id.h"
 #include "testing/gmock/include/gmock/gmock.h"
 
 namespace chromeos {
@@ -67,7 +68,7 @@ class MockAttestationFlow : public AttestationFlow {
   virtual ~MockAttestationFlow();
 
   MOCK_METHOD5(GetCertificate, void(AttestationCertificateProfile,
-                                    const std::string&,
+                                    const user_manager::UserID&,
                                     const std::string&,
                                     bool,
                                     const CertificateCallback&));