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

chromeos/dbus/mock_session_manager_client.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROMEOS_DBUS_MOCK_SESSION_MANAGER_CLIENT_H_
 #define CHROMEOS_DBUS_MOCK_SESSION_MANAGER_CLIENT_H_
 
 #include <string>
 
 #include "chromeos/dbus/session_manager_client.h"
+#include "components/user_manager/user_id.h"
 #include "testing/gmock/include/gmock/gmock.h"
 
 namespace chromeos {
 
 class MockSessionManagerClient : public SessionManagerClient {
  public:
   MockSessionManagerClient();
   virtual ~MockSessionManagerClient();
 
   MOCK_METHOD1(Init, void(dbus::Bus* bus));
   MOCK_METHOD1(SetStubDelegate, void(StubDelegate* delegate));
   MOCK_METHOD1(AddObserver, void(Observer*));
   MOCK_METHOD1(RemoveObserver, void(Observer*));
   MOCK_CONST_METHOD1(HasObserver, bool(const Observer*));
   MOCK_CONST_METHOD0(IsScreenLocked, bool(void));
   MOCK_METHOD0(EmitLoginPromptVisible, void(void));
   MOCK_METHOD2(RestartJob, void(int, const std::string&));
-  MOCK_METHOD1(StartSession, void(const std::string&));
+  MOCK_METHOD1(StartSession, void(const user_manager::UserID&));
   MOCK_METHOD0(StopSession, void(void));
   MOCK_METHOD0(NotifySupervisedUserCreationStarted, void(void));
   MOCK_METHOD0(NotifySupervisedUserCreationFinished, void(void));
   MOCK_METHOD0(StartDeviceWipe, void(void));
   MOCK_METHOD0(RequestLockScreen, void(void));
   MOCK_METHOD0(NotifyLockScreenShown, void(void));
   MOCK_METHOD0(NotifyLockScreenDismissed, void(void));
   MOCK_METHOD1(RetrieveActiveSessions, void(const ActiveSessionsCallback&));
   MOCK_METHOD1(RetrieveDevicePolicy, void(const RetrievePolicyCallback&));
   MOCK_METHOD2(RetrievePolicyForUser,
-               void(const std::string&,
+               void(const user_manager::UserID&,
                     const RetrievePolicyCallback&));
-  MOCK_METHOD1(BlockingRetrievePolicyForUser, std::string(const std::string&));
+  MOCK_METHOD1(BlockingRetrievePolicyForUser, std::string(const user_manager::UserID&));
   MOCK_METHOD2(RetrieveDeviceLocalAccountPolicy,
                void(const std::string&,
                     const RetrievePolicyCallback&));
   MOCK_METHOD2(StoreDevicePolicy,
                void(const std::string&,
                     const StorePolicyCallback&));
   MOCK_METHOD3(StorePolicyForUser,
-               void(const std::string&,
+               void(const user_manager::UserID&,
                     const std::string&,
                     const StorePolicyCallback&));
   MOCK_METHOD3(StoreDeviceLocalAccountPolicy,
                void(const std::string&,
                     const std::string&,
                     const StorePolicyCallback&));
   MOCK_METHOD2(SetFlagsForUser,
-               void(const std::string&,
+               void(const user_manager::UserID&,
                     const std::vector<std::string>&));
   MOCK_METHOD1(GetServerBackedStateKeys, void(const StateKeysCallback&));
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_MOCK_SESSION_MANAGER_CLIENT_H_