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

chromeos/dbus/mock_session_manager_client.h

Index: chromeos/dbus/mock_session_manager_client.h
diff --git a/chromeos/dbus/mock_session_manager_client.h b/chromeos/dbus/mock_session_manager_client.h
index 3a88e52d98445124d34a98775ddb70947074cfe7..a5a94d42c5ceb051efab0b2a4b2be8eddfd191a3 100644
--- a/chromeos/dbus/mock_session_manager_client.h
+++ b/chromeos/dbus/mock_session_manager_client.h
@@ -8,6 +8,7 @@
 #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 {
@@ -25,7 +26,7 @@ class MockSessionManagerClient : public SessionManagerClient {
   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));
@@ -36,9 +37,9 @@ class MockSessionManagerClient : public SessionManagerClient {
   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&));
@@ -46,7 +47,7 @@ class MockSessionManagerClient : public SessionManagerClient {
                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,
@@ -54,7 +55,7 @@ class MockSessionManagerClient : public SessionManagerClient {
                     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&));
 };