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

chromeos/attestation/attestation_flow.cc

Index: chromeos/attestation/attestation_flow.cc
diff --git a/chromeos/attestation/attestation_flow.cc b/chromeos/attestation/attestation_flow.cc
index 7cfdc87530febf39269a91d4221543d922502f96..8e0b0dabb4438809a802399cc68d444088eaefa6 100644
--- a/chromeos/attestation/attestation_flow.cc
+++ b/chromeos/attestation/attestation_flow.cc
@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "chromeos/cryptohome/async_method_caller.h"
 #include "chromeos/dbus/cryptohome_client.h"
+#include "components/user_manager/user_id.h"
 
 namespace chromeos {
 namespace attestation {
@@ -96,7 +97,7 @@ AttestationFlow::~AttestationFlow() {
 
 void AttestationFlow::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) {
@@ -193,7 +194,7 @@ void AttestationFlow::OnEnrollComplete(const base::Closure& on_failure,
 
 void AttestationFlow::StartCertificateRequest(
     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) {
@@ -245,7 +246,7 @@ void AttestationFlow::StartCertificateRequest(
 
 void AttestationFlow::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,
@@ -270,7 +271,7 @@ void AttestationFlow::SendCertificateRequestToPCA(
 
 void AttestationFlow::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,
@@ -292,7 +293,7 @@ void AttestationFlow::SendCertificateResponseToDaemon(
 
 void AttestationFlow::GetExistingCertificate(
     AttestationKeyType key_type,
-    const std::string& user_id,
+    const user_manager::UserID& user_id,
     const std::string& key_name,
     const CertificateCallback& callback) {
   cryptohome_client_->TpmAttestationGetCertificate(