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

chrome/browser/chromeos/attestation/attestation_policy_observer.cc

Index: chrome/browser/chromeos/attestation/attestation_policy_observer.cc
diff --git a/chrome/browser/chromeos/attestation/attestation_policy_observer.cc b/chrome/browser/chromeos/attestation/attestation_policy_observer.cc
index 138d8b095a86bbde7ad8fb07c51cb3cd5b12f6b5..bf6ac8e3706ae3254b1e00a785573880837abe16 100644
--- a/chrome/browser/chromeos/attestation/attestation_policy_observer.cc
+++ b/chrome/browser/chromeos/attestation/attestation_policy_observer.cc
@@ -21,6 +21,7 @@
 #include "chromeos/dbus/dbus_thread_manager.h"
 #include "components/policy/core/common/cloud/cloud_policy_client.h"
 #include "components/policy/core/common/cloud/cloud_policy_manager.h"
+#include "components/user_manager/user_id.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/notification_details.h"
 #include "net/cert/x509_certificate.h"
@@ -170,7 +171,7 @@ void AttestationPolicyObserver::Start() {
                  weak_factory_.GetWeakPtr());
   cryptohome_client_->TpmAttestationDoesKeyExist(
       KEY_DEVICE,
-      std::string(),  // Not used.
+      user_manager::EmptyUserID(),  // Not used.
       kEnterpriseMachineKey,
       base::Bind(DBusBoolRedirectCallback,
                  on_does_exist,
@@ -184,7 +185,7 @@ void AttestationPolicyObserver::GetNewCertificate() {
   // We can reuse the dbus callback handler logic.
   attestation_flow_->GetCertificate(
       PROFILE_ENTERPRISE_MACHINE_CERTIFICATE,
-      std::string(),  // Not used.
+      user_manager::EmptyUserID(),  // Not used.
       std::string(),  // Not used.
       true,  // Force a new key to be generated.
       base::Bind(DBusStringCallback,
@@ -199,7 +200,7 @@ void AttestationPolicyObserver::GetNewCertificate() {
 void AttestationPolicyObserver::GetExistingCertificate() {
   cryptohome_client_->TpmAttestationGetCertificate(
       KEY_DEVICE,
-      std::string(),  // Not used.
+      user_manager::EmptyUserID(),  // Not used.
       kEnterpriseMachineKey,
       base::Bind(DBusStringCallback,
                  base::Bind(&AttestationPolicyObserver::CheckCertificateExpiry,
@@ -257,7 +258,7 @@ void AttestationPolicyObserver::GetKeyPayload(
     base::Callback<void(const std::string&)> callback) {
   cryptohome_client_->TpmAttestationGetKeyPayload(
       KEY_DEVICE,
-      std::string(),  // Not used.
+      user_manager::EmptyUserID(),  // Not used.
       kEnterpriseMachineKey,
       base::Bind(DBusStringCallback,
                  callback,
@@ -286,7 +287,7 @@ void AttestationPolicyObserver::MarkAsUploaded(const std::string& key_payload) {
   }
   cryptohome_client_->TpmAttestationSetKeyPayload(
       KEY_DEVICE,
-      std::string(),  // Not used.
+      user_manager::EmptyUserID(),  // Not used.
       kEnterpriseMachineKey,
       new_payload,
       base::Bind(DBusBoolRedirectCallback,