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

chrome/browser/chromeos/login/user_flow.cc

Index: chrome/browser/chromeos/login/user_flow.cc
diff --git a/chrome/browser/chromeos/login/user_flow.cc b/chrome/browser/chromeos/login/user_flow.cc
index bff2445fd556f962b6ee17c5fdc758fca4564f71..1b9c5f564fe093ae12e20713b9e3a58dacd05519 100644
--- a/chrome/browser/chromeos/login/user_flow.cc
+++ b/chrome/browser/chromeos/login/user_flow.cc
@@ -11,7 +11,7 @@ namespace chromeos {
 
 namespace {
 
-void UnregisterFlow(const std::string& user_id) {
+void UnregisterFlow(const user_manager::UserID& user_id) {
   ChromeUserManager::Get()->ResetUserFlow(user_id);
 }
 
@@ -70,7 +70,7 @@ void DefaultUserFlow::HandleOAuthTokenStatusChange(
 void DefaultUserFlow::LaunchExtraSteps(Profile* profile) {
 }
 
-ExtendedUserFlow::ExtendedUserFlow(const std::string& user_id)
+ExtendedUserFlow::ExtendedUserFlow(const user_manager::UserID& user_id)
     : user_id_(user_id) {
 }
 
@@ -89,7 +89,7 @@ void ExtendedUserFlow::HandleOAuthTokenStatusChange(
 }
 
 void ExtendedUserFlow::UnregisterFlowSoon() {
-  std::string id_copy(user_id());
+  user_manager::UserID id_copy(user_id());
   base::MessageLoop::current()->PostTask(FROM_HERE,
       base::Bind(&UnregisterFlow,
                  id_copy));