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

chrome/browser/ui/webui/options/create_profile_handler.cc

Index: chrome/browser/ui/webui/options/create_profile_handler.cc
diff --git a/chrome/browser/ui/webui/options/create_profile_handler.cc b/chrome/browser/ui/webui/options/create_profile_handler.cc
index ef749c8c69427164ee11fb01c391cceed8586c15..ce490c27be0b851bfc54fc7976aff14d93887a88 100644
--- a/chrome/browser/ui/webui/options/create_profile_handler.cc
+++ b/chrome/browser/ui/webui/options/create_profile_handler.cc
@@ -93,7 +93,7 @@ void CreateProfileHandler::CreateProfile(const base::ListValue* args) {
     CHECK(!name.empty());
     args->GetBoolean(2, &create_shortcut);
   }
-  std::string supervised_user_id;
+  user_manager::UserID supervised_user_id = user_manager::UserID(std::string(), std::string());
 #if defined(ENABLE_SUPERVISED_USERS)
   if (!ProcessSupervisedCreateProfileArgs(args, &supervised_user_id))
     return;
@@ -114,7 +114,7 @@ void CreateProfileHandler::CreateProfile(const base::ListValue* args) {
 void CreateProfileHandler::OnProfileCreated(
     bool create_shortcut,
     chrome::HostDesktopType desktop_type,
-    const std::string& supervised_user_id,
+    const user_manager::UserID& supervised_user_id,
     Profile* profile,
     Profile::CreateStatus status) {
   if (status != Profile::CREATE_STATUS_CREATED)
@@ -150,7 +150,7 @@ void CreateProfileHandler::OnProfileCreated(
 void CreateProfileHandler::HandleProfileCreationSuccess(
     bool create_shortcut,
     chrome::HostDesktopType desktop_type,
-    const std::string& supervised_user_id,
+    const user_manager::UserID& supervised_user_id,
     Profile* profile) {
   switch (profile_creation_type_) {
     case NON_SUPERVISED_PROFILE_CREATION: {
@@ -302,14 +302,16 @@ std::string CreateProfileHandler::GetJavascriptMethodName(
 
 #if defined(ENABLE_SUPERVISED_USERS)
 bool CreateProfileHandler::ProcessSupervisedCreateProfileArgs(
-    const base::ListValue* args, std::string* supervised_user_id) {
+    const base::ListValue* args, user_manager::UserID* supervised_user_id) {
   bool supervised_user = false;
   if (args->GetSize() >= 5) {
       bool success = args->GetBoolean(3, &supervised_user);
       DCHECK(success);
 
-      success = args->GetString(4, supervised_user_id);
+      std::string supervised_user_email;
+      success = args->GetString(4, &supervised_user_email);
       DCHECK(success);
+      *(supervised_user_id) = user_manager::UserID::FromUserEmail(supervised_user_email);
   }
 
   if (supervised_user) {
@@ -378,7 +380,7 @@ void CreateProfileHandler::CancelProfileRegistration(bool user_initiated) {
 void CreateProfileHandler::RegisterSupervisedUser(
     bool create_shortcut,
     chrome::HostDesktopType desktop_type,
-    const std::string& supervised_user_id,
+    const user_manager::UserID& supervised_user_id,
     Profile* new_profile) {
   DCHECK_EQ(profile_path_being_created_.value(),
             new_profile->GetPath().value());
@@ -452,7 +454,7 @@ void CreateProfileHandler::RecordSupervisedProfileCreationMetrics(
 }
 
 bool CreateProfileHandler::IsValidExistingSupervisedUserId(
-    const std::string& existing_supervised_user_id) const {
+    const user_manager::UserID& existing_supervised_user_id) const {
   if (existing_supervised_user_id.empty())
     return true;
 
@@ -460,7 +462,7 @@ bool CreateProfileHandler::IsValidExistingSupervisedUserId(
   const base::DictionaryValue* dict =
       SupervisedUserSyncServiceFactory::GetForProfile(profile)->
           GetSupervisedUsers();
-  if (!dict->HasKey(existing_supervised_user_id))
+  if (!dict->HasKey(existing_supervised_user_id.GetUserEmail()))
     return false;
 
   // Check if this supervised user already exists on this machine.