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

chrome/browser/chromeos/login/users/mock_user_manager.h

Index: chrome/browser/chromeos/login/users/mock_user_manager.h
diff --git a/chrome/browser/chromeos/login/users/mock_user_manager.h b/chrome/browser/chromeos/login/users/mock_user_manager.h
index c7545bc6b6e7dcfbf8a7c085e3fb82fc20e23868..9373787c734a5d6e1a2345169030135bc47bdd0d 100644
--- a/chrome/browser/chromeos/login/users/mock_user_manager.h
+++ b/chrome/browser/chromeos/login/users/mock_user_manager.h
@@ -110,7 +110,7 @@ class MockUserManager : public ChromeUserManager {
   // ChromeUserManager overrides:
   BootstrapManager* GetBootstrapManager() override;
   MultiProfileUserController* GetMultiProfileUserController() override;
-  UserImageManager* GetUserImageManager(const std::string& user_id) override;
+  UserImageManager* GetUserImageManager(const user_manager::UserID& user_id) override;
   SupervisedUserManager* GetSupervisedUserManager() override;
   MOCK_METHOD2(SetUserFlow, void(const std::string&, UserFlow*));
   MOCK_METHOD1(ResetUserFlow, void(const std::string&));
@@ -119,15 +119,15 @@ class MockUserManager : public ChromeUserManager {
 
   // Sets a new User instance. Users previously created by this MockUserManager
   // become invalid.
-  void SetActiveUser(const std::string& email);
+  void SetActiveUser(const user_manager::UserID& user_id);
 
   // Creates a new public session user. Users previously created by this
   // MockUserManager become invalid.
-  user_manager::User* CreatePublicAccountUser(const std::string& email);
+  user_manager::User* CreatePublicAccountUser(const user_manager::UserID& user_id);
 
   // Creates a new kiosk app user. Users previously created by this
   // MockUserManager become invalid.
-  user_manager::User* CreateKioskAppUser(const std::string& user_id);
+  user_manager::User* CreateKioskAppUser(const user_manager::UserID& user_id);
 
   // Adds a new User instance to the back of the user list. Users previously
   // created by this MockUserManager remain valid.