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

chrome/browser/ui/ash/multi_user/multi_user_util.cc

Index: chrome/browser/ui/ash/multi_user/multi_user_util.cc
diff --git a/chrome/browser/ui/ash/multi_user/multi_user_util.cc b/chrome/browser/ui/ash/multi_user/multi_user_util.cc
index e3444a283a87777a34a52d193ab08d137f71e6b2..ce5f01bf81726b77435d074235d7d704ac4cdd4d 100644
--- a/chrome/browser/ui/ash/multi_user/multi_user_util.cc
+++ b/chrome/browser/ui/ash/multi_user/multi_user_util.cc
@@ -10,6 +10,7 @@
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
+#include "components/user_manager/user_id.h"
 #include "google_apis/gaia/gaia_auth_util.h"
 
 #if defined(OS_CHROMEOS)
@@ -19,18 +20,18 @@
 
 namespace multi_user_util {
 
-std::string GetUserIDFromProfile(Profile* profile) {
+user_manager::UserID GetUserIDFromProfile(Profile* profile) {
   return GetUserIDFromEmail(
       profile->GetOriginalProfile()->GetProfileUserName());
 }
 
-std::string GetUserIDFromEmail(const std::string& email) {
+user_manager::UserID GetUserIDFromEmail(const std::string& email) {
   // |email| and profile name could be empty if not yet logged in or guest mode.
-  return email.empty() ?
-      email : gaia::CanonicalizeEmail(gaia::SanitizeEmail(email));
+  return user_manager::UserID::FromUserEmail(email.empty() ?
+      email : gaia::CanonicalizeEmail(gaia::SanitizeEmail(email)));
 }
 
-Profile* GetProfileFromUserID(const std::string& user_id) {
+Profile* GetProfileFromUserID(const user_manager::UserID& user_id) {
   // Unit tests can end up here without a |g_browser_process|.
   if (!g_browser_process || !g_browser_process->profile_manager())
     return NULL;
@@ -53,7 +54,7 @@ Profile* GetProfileFromWindow(aura::Window* window) {
   // We might come here before the manager got created - or in a unit test.
   if (!manager)
     return NULL;
-  const std::string user_id = manager->GetUserPresentingWindow(window);
+  const user_manager::UserID user_id = manager->GetUserPresentingWindow(window);
   return user_id.empty() ? NULL :
                            multi_user_util::GetProfileFromUserID(user_id);
 #else
@@ -64,7 +65,7 @@ Profile* GetProfileFromWindow(aura::Window* window) {
 bool IsProfileFromActiveUser(Profile* profile) {
 #if defined(OS_CHROMEOS)
   return GetUserIDFromProfile(profile) ==
-         user_manager::UserManager::Get()->GetActiveUser()->email();
+         user_manager::UserManager::Get()->GetActiveUser()->GetUserID();
 #else
   // In non Chrome OS configurations this will be always true since this only
   // makes sense in separate desktop mode.
@@ -72,11 +73,11 @@ bool IsProfileFromActiveUser(Profile* profile) {
 #endif
 }
 
-const std::string& GetCurrentUserId() {
+const user_manager::UserID& GetCurrentUserId() {
 #if defined(OS_CHROMEOS)
-  return user_manager::UserManager::Get()->GetActiveUser()->email();
+  return user_manager::UserManager::Get()->GetActiveUser()->GetUserID();
 #else
-  return base::EmptyString();
+  return user_manager::UserID(std::string(), std::string());
 #endif
 }