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

chrome/browser/chromeos/login/users/fake_supervised_user_manager.cc

Index: chrome/browser/chromeos/login/users/fake_supervised_user_manager.cc
diff --git a/chrome/browser/chromeos/login/users/fake_supervised_user_manager.cc b/chrome/browser/chromeos/login/users/fake_supervised_user_manager.cc
index a70f31be9e7be09f2bb00598f4d8827adc174a0b..321d845459506c7313f1f17f27759aebca791cb9 100644
--- a/chrome/browser/chromeos/login/users/fake_supervised_user_manager.cc
+++ b/chrome/browser/chromeos/login/users/fake_supervised_user_manager.cc
@@ -19,15 +19,15 @@ bool FakeSupervisedUserManager::HasSupervisedUsers(
 }
 
 const user_manager::User* FakeSupervisedUserManager::CreateUserRecord(
-    const std::string& manager_id,
-    const std::string& local_user_id,
+    const user_manager::UserID& manager_id,
+    const user_manager::UserID& local_user_id,
     const std::string& sync_user_id,
     const base::string16& display_name) {
   return NULL;
 }
 
-std::string FakeSupervisedUserManager::GenerateUserId() {
-  return std::string();
+user_manager::UserID FakeSupervisedUserManager::GenerateUserId() {
+  returnuser_manager::UserID(std::string() /* gaia_id */, std::string() /* user_email */);

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:45]

Typo (space is missing), does it compile???

 }
 
 const user_manager::User* FakeSupervisedUserManager::FindByDisplayName(
@@ -41,7 +41,7 @@ const user_manager::User* FakeSupervisedUserManager::FindBySyncId(
 }
 
 std::string FakeSupervisedUserManager::GetUserSyncId(
-    const std::string& supervised_user_id) const {
+    const user_manager::UserID& supervised_user_id) const {
   return std::string();
 }
 
@@ -50,13 +50,13 @@ base::string16 FakeSupervisedUserManager::GetManagerDisplayName(
   return base::string16();
 }
 
-std::string FakeSupervisedUserManager::GetManagerUserId(
-    const std::string& supervised_user_id) const {
-  return std::string();
+const UserID& FakeSupervisedUserManager::GetManagerUserId(
+    const UserID& supervised_user_id) const {
+  return EmptyUserID();
 }
 
 std::string FakeSupervisedUserManager::GetManagerDisplayEmail(
-    const std::string& supervised_user_id) const {
+    const user_manager::UserID& supervised_user_id) const {
   return std::string();
 }