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

chromeos/attestation/attestation_flow.h

Index: chromeos/attestation/attestation_flow.h
diff --git a/chromeos/attestation/attestation_flow.h b/chromeos/attestation/attestation_flow.h
index a6c6cd22d39fde66a7300358bfdfc7e3838de8e7..bc44d17e992ecaf5b1c6a93ecce1aa2a719a467d 100644
--- a/chromeos/attestation/attestation_flow.h
+++ b/chromeos/attestation/attestation_flow.h
@@ -22,6 +22,10 @@ class AsyncMethodCaller;
 
 }  // namespace cryptohome
 
+namespace  user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class CryptohomeClient;
@@ -82,7 +86,7 @@ class CHROMEOS_EXPORT AttestationFlow {
   //              On success |result| will be true and |data| will contain the
   //              PCA-issued certificate chain in PEM format.
   virtual void GetCertificate(AttestationCertificateProfile certificate_profile,
-                              const std::string& user_id,
+                              const user_manager::UserID& user_id,
                               const std::string& request_origin,
                               bool force_new_key,
                               const CertificateCallback& callback);
@@ -149,7 +153,7 @@ class CHROMEOS_EXPORT AttestationFlow {
   //   callback - Called when the operation completes.
   void StartCertificateRequest(
       const AttestationCertificateProfile certificate_profile,
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       const std::string& request_origin,
       bool generate_new_key,
       const CertificateCallback& callback);
@@ -166,7 +170,7 @@ class CHROMEOS_EXPORT AttestationFlow {
   //   success - The status of request creation.
   //   data - The request data for the Privacy CA.
   void SendCertificateRequestToPCA(AttestationKeyType key_type,
-                                   const std::string& user_id,
+                                   const user_manager::UserID& user_id,
                                    const std::string& key_name,
                                    const CertificateCallback& callback,
                                    bool success,
@@ -184,7 +188,7 @@ class CHROMEOS_EXPORT AttestationFlow {
   //   success - The status of the Privacy CA operation.
   //   data - The response data from the Privacy CA.
   void SendCertificateResponseToDaemon(AttestationKeyType key_type,
-                                       const std::string& user_id,
+                                       const user_manager::UserID& user_id,
                                        const std::string& key_name,
                                        const CertificateCallback& callback,
                                        bool success,
@@ -198,7 +202,7 @@ class CHROMEOS_EXPORT AttestationFlow {
   //   key_name - The name of the key for which a certificate is requested.
   //   callback - Called when the operation completes.
   void GetExistingCertificate(AttestationKeyType key_type,
-                              const std::string& user_id,
+                              const user_manager::UserID& user_id,
                               const std::string& key_name,
                               const CertificateCallback& callback);