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

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

Index: chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.cc
diff --git a/chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.cc b/chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.cc
index 1cfd624486b1664ea1da3f182f07c07480053346..32182749f3bc0356f906b27d4abf2080f8f46b00 100644
--- a/chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.cc
+++ b/chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.cc
@@ -216,7 +216,7 @@ void SupervisedUserCreationScreen::HideFlow() {
 }
 
 void SupervisedUserCreationScreen::AuthenticateManager(
-    const std::string& manager_id,
+    const user_manager::UserID& manager_id,
     const std::string& manager_password) {
   if (manager_signin_in_progress_)
     return;
@@ -250,13 +250,13 @@ void SupervisedUserCreationScreen::CreateSupervisedUser(
 }
 
 void SupervisedUserCreationScreen::ImportSupervisedUser(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   DCHECK(controller_.get());
   DCHECK(existing_users_.get());
-  VLOG(1) << "Importing user " << user_id;
+  VLOG(1) << "Importing user " << user_id.GetUserEmail();
   base::DictionaryValue* user_info;
-  if (!existing_users_->GetDictionary(user_id, &user_info)) {
-    LOG(ERROR) << "Can not import non-existing user " << user_id;
+  if (!existing_users_->GetDictionary(user_id.GetUserEmail(), &user_info)) {
+    LOG(ERROR) << "Can not import non-existing user " << user_id.GetUserEmail();
     return;
   }
   base::string16 display_name;
@@ -294,7 +294,7 @@ void SupervisedUserCreationScreen::ImportSupervisedUser(
       SupervisedUserSharedSettingsServiceFactory::GetForBrowserContext(
           controller_->GetManagerProfile());
   const base::Value* value = shared_settings_service->GetValue(
-      user_id, supervised_users::kChromeOSPasswordData);
+      user_id.GetUserEmail(), supervised_users::kChromeOSPasswordData);
 
   bool password_right_here = value && value->GetAsDictionary(&password_data) &&
                              !password_data->empty();
@@ -302,7 +302,7 @@ void SupervisedUserCreationScreen::ImportSupervisedUser(
   if (password_right_here) {
     controller_->StartImport(display_name,
                              avatar_index,
-                             user_id,
+                             user_id.GetUserEmail(),
                              master_key,
                              password_data,
                              encryption_key,
@@ -315,14 +315,14 @@ void SupervisedUserCreationScreen::ImportSupervisedUser(
 // TODO(antrim): Code duplication with previous method will be removed once
 // password sync is implemented.
 void SupervisedUserCreationScreen::ImportSupervisedUserWithPassword(
-    const std::string& user_id,
+    const user_manager::UserID& user_id,
     const std::string& password) {
   DCHECK(controller_.get());
   DCHECK(existing_users_.get());
-  VLOG(1) << "Importing user " << user_id;
+  VLOG(1) << "Importing user " << user_id.GetUserEmail();
   base::DictionaryValue* user_info;
-  if (!existing_users_->GetDictionary(user_id, &user_info)) {
-    LOG(ERROR) << "Can not import non-existing user " << user_id;
+  if (!existing_users_->GetDictionary(user_id.GetUserEmail(), &user_info)) {
+    LOG(ERROR) << "Can not import non-existing user " << user_id.GetUserEmail();
     return;
   }
   base::string16 display_name;
@@ -352,7 +352,7 @@ void SupervisedUserCreationScreen::ImportSupervisedUserWithPassword(
   controller_->StartImport(display_name,
                            password,
                            avatar_index,
-                           user_id,
+                           user_id.GetUserEmail(),
                            master_key);
 }
 
@@ -458,7 +458,7 @@ void SupervisedUserCreationScreen::OnLongCreationWarning() {
 
 bool SupervisedUserCreationScreen::FindUserByDisplayName(
     const base::string16& display_name,
-    std::string *out_id) const {
+    std::string *out_email) const {
   if (!existing_users_.get())
     return false;
   for (base::DictionaryValue::Iterator it(*existing_users_.get());
@@ -469,8 +469,8 @@ bool SupervisedUserCreationScreen::FindUserByDisplayName(
     if (user_info->GetString(SupervisedUserSyncService::kName,
                              &user_display_name)) {
       if (display_name == user_display_name) {
-        if (out_id)
-          *out_id = it.key();
+        if (out_email)
+          *out_email = it.key();
         return true;
       }
     }
@@ -482,7 +482,7 @@ bool SupervisedUserCreationScreen::FindUserByDisplayName(
 // It should be removed by issue 251179.
 
 void SupervisedUserCreationScreen::ApplyPicture() {
-  std::string user_id = controller_->GetSupervisedUserId();
+  user_manager::UserID user_id = controller_->GetSupervisedUserId();
   UserImageManager* image_manager =
       ChromeUserManager::Get()->GetUserImageManager(user_id);
   switch (selected_image_) {