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

chrome/browser/chromeos/login/supervised/supervised_user_authentication.cc

Index: chrome/browser/chromeos/login/supervised/supervised_user_authentication.cc
diff --git a/chrome/browser/chromeos/login/supervised/supervised_user_authentication.cc b/chrome/browser/chromeos/login/supervised/supervised_user_authentication.cc
index b4a64a35b45edd4d75d1790465b6a74a07777263..e0ceb8f1ade6e55b72d737e9553a04b1d44d1810 100644
--- a/chrome/browser/chromeos/login/supervised/supervised_user_authentication.cc
+++ b/chrome/browser/chromeos/login/supervised/supervised_user_authentication.cc
@@ -121,12 +121,12 @@ UserContext SupervisedUserAuthentication::TransformKey(
     result.SetIsUsingOAuth(false);
     return result;
   }
-  NOTREACHED() << "Unknown password schema for " << context.GetUserID();
+  NOTREACHED() << "Unknown password schema for " << context.GetUserID().GetUserEmail();
   return context;
 }
 
 bool SupervisedUserAuthentication::FillDataForNewUser(
-    const std::string& user_id,
+    const user_manager::UserID& /* user_id */,
     const std::string& password,
     base::DictionaryValue* password_data,
     base::DictionaryValue* extra_data) {
@@ -170,7 +170,7 @@ std::string SupervisedUserAuthentication::GenerateMasterKey() {
 }
 
 void SupervisedUserAuthentication::StorePasswordData(
-    const std::string& user_id,
+    const user_manager::UserID& user_id,
     const base::DictionaryValue& password_data) {
   base::DictionaryValue holder;
   owner_->GetPasswordInformation(user_id, &holder);
@@ -186,7 +186,7 @@ void SupervisedUserAuthentication::StorePasswordData(
 
 SupervisedUserAuthentication::Schema
 SupervisedUserAuthentication::GetPasswordSchema(
-  const std::string& user_id) {
+  const user_manager::UserID& user_id) {
   base::DictionaryValue holder;
 
   owner_->GetPasswordInformation(user_id, &holder);
@@ -201,7 +201,7 @@ SupervisedUserAuthentication::GetPasswordSchema(
 }
 
 bool SupervisedUserAuthentication::NeedPasswordChange(
-    const std::string& user_id,
+    const user_manager::UserID& user_id,
     const base::DictionaryValue* password_data) {
   base::DictionaryValue local;
   owner_->GetPasswordInformation(user_id, &local);
@@ -222,7 +222,7 @@ bool SupervisedUserAuthentication::NeedPasswordChange(
 }
 
 void SupervisedUserAuthentication::ScheduleSupervisedPasswordChange(
-    const std::string& supervised_user_id,
+    const user_manager::UserID& supervised_user_id,
     const base::DictionaryValue* password_data) {
   const user_manager::User* user =
       user_manager::UserManager::Get()->FindUser(supervised_user_id);
@@ -231,7 +231,7 @@ void SupervisedUserAuthentication::ScheduleSupervisedPasswordChange(
   JSONFileValueSerializer serializer(profile_path.Append(kPasswordUpdateFile));
   if (!serializer.Serialize(*password_data)) {
     LOG(ERROR) << "Failed to schedule password update for supervised user "
-               << supervised_user_id;
+               << supervised_user_id.GetUserEmail();
     UMA_HISTOGRAM_ENUMERATION(
         "ManagedUsers.ChromeOS.PasswordChange",
         SupervisedUserAuthentication::PASSWORD_CHANGE_FAILED_STORE_DATA,
@@ -245,7 +245,7 @@ void SupervisedUserAuthentication::ScheduleSupervisedPasswordChange(
 }
 
 bool SupervisedUserAuthentication::HasScheduledPasswordUpdate(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   base::DictionaryValue holder;
   owner_->GetPasswordInformation(user_id, &holder);
   bool require_update = false;
@@ -254,7 +254,7 @@ bool SupervisedUserAuthentication::HasScheduledPasswordUpdate(
 }
 
 void SupervisedUserAuthentication::ClearScheduledPasswordUpdate(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   base::DictionaryValue holder;
   owner_->GetPasswordInformation(user_id, &holder);
   holder.SetBoolean(kRequirePasswordUpdate, false);
@@ -262,7 +262,7 @@ void SupervisedUserAuthentication::ClearScheduledPasswordUpdate(
 }
 
 bool SupervisedUserAuthentication::HasIncompleteKey(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   base::DictionaryValue holder;
   owner_->GetPasswordInformation(user_id, &holder);
   bool incomplete_key = false;
@@ -270,7 +270,7 @@ bool SupervisedUserAuthentication::HasIncompleteKey(
   return incomplete_key;
 }
 
-void SupervisedUserAuthentication::MarkKeyIncomplete(const std::string& user_id,
+void SupervisedUserAuthentication::MarkKeyIncomplete(const user_manager::UserID& user_id,
                                                      bool incomplete) {
   base::DictionaryValue holder;
   owner_->GetPasswordInformation(user_id, &holder);
@@ -279,7 +279,7 @@ void SupervisedUserAuthentication::MarkKeyIncomplete(const std::string& user_id,
 }
 
 void SupervisedUserAuthentication::LoadPasswordUpdateData(
-    const std::string& user_id,
+    const user_manager::UserID& user_id,
     const PasswordDataCallback& success_callback,
     const base::Closure& failure_callback) {
   const user_manager::User* user =