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

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

Index: chrome/browser/chromeos/login/users/fake_chrome_user_manager.h
diff --git a/chrome/browser/chromeos/login/users/fake_chrome_user_manager.h b/chrome/browser/chromeos/login/users/fake_chrome_user_manager.h
index 238f21c973e27a064968089d517f96ad8b4ffd76..e0120371eb7e713d2e6f757bb712a78a4a781b5c 100644
--- a/chrome/browser/chromeos/login/users/fake_chrome_user_manager.h
+++ b/chrome/browser/chromeos/login/users/fake_chrome_user_manager.h
@@ -13,6 +13,7 @@
 #include "chrome/browser/chromeos/login/users/user_manager_interface.h"
 #include "components/user_manager/fake_user_manager.h"
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 #include "components/user_manager/user_image/user_image.h"
 #include "components/user_manager/user_manager_base.h"
 
@@ -29,35 +30,35 @@ class FakeChromeUserManager : public user_manager::FakeUserManager,
   ~FakeChromeUserManager() override;
 
   // Create and add a kiosk app user.
-  void AddKioskAppUser(const std::string& kiosk_app_username);
+  void AddKioskAppUser(const user_manager::UserID& kiosk_app);
 
   // Create and add a public account user.
-  const user_manager::User* AddPublicAccountUser(const std::string& email);
+  const user_manager::User* AddPublicAccountUser(const user_manager::UserID& user_id);
 
   // Calculates the user name hash and calls UserLoggedIn to login a user.
-  void LoginUser(const std::string& email);
+  void LoginUser(const user_manager::UserID& user_id);
 
   // UserManager overrides.
   user_manager::UserList GetUsersAllowedForMultiProfile() const override;
 
   // user_manager::FakeUserManager override.
-  const user_manager::User* AddUser(const std::string& email) override;
+  const user_manager::User* AddUser(const user_manager::UserID& user_id) override;
 
   // UserManagerInterface implementation.
   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;
-  void SetUserFlow(const std::string& email, UserFlow* flow) override;
+  void SetUserFlow(const user_manager::UserID& user_id, UserFlow* flow) override;
   UserFlow* GetCurrentUserFlow() const override;
-  UserFlow* GetUserFlow(const std::string& email) const override;
-  void ResetUserFlow(const std::string& email) override;
+  UserFlow* GetUserFlow(const user_manager::UserID& user_id) const override;
+  void ResetUserFlow(const user_manager::UserID& user_id) override;
   user_manager::UserList GetUsersAllowedForSupervisedUsersCreation()
       const override;
-  void SwitchActiveUser(const std::string& email) override;
-  const std::string& GetOwnerEmail() const override;
+  void SwitchActiveUser(const user_manager::UserID& user_id) override;
+  const user_manager::UserID& GetOwnerID() const override;
   void SessionStarted() override;
-  void RemoveUser(const std::string& email,
+  void RemoveUser(const user_manager::UserID& user_id,
                   user_manager::RemoveUserDelegate* delegate) override;
   bool FindKnownUserPrefs(const user_manager::UserID& user_id,
                           const base::DictionaryValue** out_value) override;
@@ -65,8 +66,8 @@ class FakeChromeUserManager : public user_manager::FakeUserManager,
                             const base::DictionaryValue& values,
                             bool clear) override;
 
-  void set_owner_email(const std::string& owner_email) {
-    owner_email_ = owner_email;
+  void set_owner_id(const user_manager::UserID& owner_id) {
+    owner_id_ = owner_id;
   }
 
   void set_bootstrap_manager(BootstrapManager* bootstrap_manager) {
@@ -83,12 +84,12 @@ class FakeChromeUserManager : public user_manager::FakeUserManager,
   UserFlow* GetDefaultUserFlow() const;
 
   scoped_ptr<FakeSupervisedUserManager> supervised_user_manager_;
-  std::string owner_email_;
+  user_manager::UserID owner_id_;
 
   BootstrapManager* bootstrap_manager_;
   MultiProfileUserController* multi_profile_user_controller_;
 
-  typedef std::map<std::string, UserFlow*> FlowMap;
+  typedef std::map<user_manager::UserID, UserFlow*> FlowMap;
 
   // Lazy-initialized default flow.
   mutable scoped_ptr<UserFlow> default_flow_;