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

chrome/browser/profiles/profile_window.cc

Index: chrome/browser/profiles/profile_window.cc
diff --git a/chrome/browser/profiles/profile_window.cc b/chrome/browser/profiles/profile_window.cc
index 177bb3bf4415fce717945152c04d62ff3ac59030..846d1068f01bab301cd33d5191d05530111765f2 100644
--- a/chrome/browser/profiles/profile_window.cc
+++ b/chrome/browser/profiles/profile_window.cc
@@ -288,7 +288,7 @@ void SwitchToProfile(const base::FilePath& path,
                  desktop_type),
       base::string16(),
       base::string16(),
-      std::string());
+      user_manager::UserID(std::string(), std::string()));

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:47]

EmptyUserID

 }
 
 void SwitchToGuestProfile(chrome::HostDesktopType desktop_type,
@@ -306,7 +306,7 @@ void SwitchToGuestProfile(chrome::HostDesktopType desktop_type,
                  desktop_type),
       base::string16(),
       base::string16(),
-      std::string());
+      user_manager::UserID(std::string(), std::string()));
 }
 
 bool HasProfileSwitchTargets(Profile* profile) {
@@ -332,7 +332,7 @@ void CreateAndSwitchToNewProfile(chrome::HostDesktopType desktop_type,
                  true,
                  true,
                  desktop_type),
-      std::string());
+      user_manager::UserID(std::string(), std::string()));
   ProfileMetrics::LogProfileAddNewUser(metric);
 }
 
@@ -430,7 +430,7 @@ void CreateSystemProfileForUserManager(
                  callback),
       base::string16(),
       base::string16(),
-      std::string());
+      user_manager::UserID(std::string(), std::string()));
 }
 
 void ShowUserManagerMaybeWithTutorial(Profile* profile) {