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

components/user_manager/user.cc

Index: components/user_manager/user.cc
diff --git a/components/user_manager/user.cc b/components/user_manager/user.cc
index 6a7dec1ab8b6fb23c7f30bbfc6d94bbc1821af24..9d5febef85a906ee7e1a851c62d3b80d68ea09a8 100644
--- a/components/user_manager/user.cc
+++ b/components/user_manager/user.cc
@@ -37,7 +37,7 @@ bool User::TypeHasGaiaAccount(UserType user_type) {
 // Also used for regular supervised users.
 class RegularUser : public User {
  public:
-  explicit RegularUser(const std::string& email);
+  explicit RegularUser(const UserID& user_id);
   ~RegularUser() override;
 
   // Overridden from User:
@@ -65,7 +65,7 @@ class GuestUser : public User {
 
 class KioskAppUser : public User {
  public:
-  explicit KioskAppUser(const std::string& app_id);
+  explicit KioskAppUser(const UserID& app_id);
   ~KioskAppUser() override;
 
   // Overridden from User:
@@ -77,7 +77,7 @@ class KioskAppUser : public User {
 
 class SupervisedUser : public User {
  public:
-  explicit SupervisedUser(const std::string& username);
+  explicit SupervisedUser(const UserID& user_id);
   ~SupervisedUser() override;
 
   // Overridden from User:
@@ -90,7 +90,7 @@ class SupervisedUser : public User {
 
 class PublicAccountUser : public User {
  public:
-  explicit PublicAccountUser(const std::string& email);
+  explicit PublicAccountUser(const UserID& user_id);
   ~PublicAccountUser() override;
 
   // Overridden from User:
@@ -118,8 +118,9 @@ const gfx::ImageSkia& User::GetImage() const {
   return user_image_.image();
 }
 
-UserID User::GetUserID() const {
-  return gaia::CanonicalizeEmail(gaia::SanitizeEmail(email()));
+const UserID& User::GetUserID() const {
+  return user_id_;
+  //return UserID(std::string() /* gaia_id */, gaia::CanonicalizeEmail(gaia::SanitizeEmail(email())));
 }
 
 void User::SetIsChild(bool is_child) {
@@ -144,7 +145,7 @@ std::string User::GetAccountName(bool use_display_email) const {
   if (use_display_email && !display_email_.empty())
     return GetUserName(display_email_);
   else
-    return GetUserName(email_);
+    return GetUserName(user_id_.GetUserEmail());
 }
 
 bool User::HasDefaultImage() const {
@@ -175,28 +176,28 @@ bool User::is_active() const {
   return is_active_;
 }
 
-User* User::CreateRegularUser(const std::string& email) {
-  return new RegularUser(email);
+User* User::CreateRegularUser(const UserID& user_id) {
+  return new RegularUser(user_id);
 }
 
 User* User::CreateGuestUser() {
   return new GuestUser;
 }
 
-User* User::CreateKioskAppUser(const std::string& kiosk_app_username) {
-  return new KioskAppUser(kiosk_app_username);
+User* User::CreateKioskAppUser(const UserID& user_id) {
+  return new KioskAppUser(user_id);
 }
 
-User* User::CreateSupervisedUser(const std::string& username) {
-  return new SupervisedUser(username);
+User* User::CreateSupervisedUser(const UserID& user_id) {
+  return new SupervisedUser(user_id);
 }
 
-User* User::CreatePublicAccountUser(const std::string& email) {
-  return new PublicAccountUser(email);
+User* User::CreatePublicAccountUser(const UserID& user_id) {
+  return new PublicAccountUser(user_id);
 }
 
-User::User(const std::string& email)
-    : email_(email),
+User::User(const UserID& user_id)
+    : user_id_(user_id),
       oauth_token_status_(OAUTH_TOKEN_STATUS_UNKNOWN),
       force_online_signin_(false),
       image_index_(USER_IMAGE_INVALID),
@@ -236,10 +237,10 @@ void User::SetStubImage(const UserImage& stub_user_image,
   image_is_loading_ = is_loading;
 }
 
-RegularUser::RegularUser(const std::string& email)
-    : User(email), is_child_(false) {
+RegularUser::RegularUser(const UserID& user_id)
+    : User(user_id), is_child_(false) {
   set_can_lock(true);
-  set_display_email(email);
+  set_display_email(user_id.GetUserEmail());
 }
 
 RegularUser::~RegularUser() {
@@ -259,7 +260,7 @@ void RegularUser::SetIsChild(bool is_child) {
   is_child_ = is_child;
 }
 
-GuestUser::GuestUser() : User(chromeos::login::kGuestUserName) {
+GuestUser::GuestUser() : User(chromeos::login::GetGuestUserID()) {
   set_display_email(std::string());
 }
 
@@ -270,9 +271,9 @@ UserType GuestUser::GetType() const {
   return user_manager::USER_TYPE_GUEST;
 }
 
-KioskAppUser::KioskAppUser(const std::string& kiosk_app_username)
-    : User(kiosk_app_username) {
-  set_display_email(kiosk_app_username);
+KioskAppUser::KioskAppUser(const UserID& kiosk_app)
+    : User(kiosk_app) {
+  set_display_email(kiosk_app.GetUserEmail());
 }
 
 KioskAppUser::~KioskAppUser() {
@@ -282,7 +283,7 @@ UserType KioskAppUser::GetType() const {
   return user_manager::USER_TYPE_KIOSK_APP;
 }
 
-SupervisedUser::SupervisedUser(const std::string& username) : User(username) {
+SupervisedUser::SupervisedUser(const UserID& user_id) : User(user_id) {
   set_can_lock(true);
 }
 
@@ -297,7 +298,7 @@ std::string SupervisedUser::display_email() const {
   return base::UTF16ToUTF8(display_name());
 }
 
-PublicAccountUser::PublicAccountUser(const std::string& email) : User(email) {
+PublicAccountUser::PublicAccountUser(const UserID& user_id) : User(user_id) {
 }
 
 PublicAccountUser::~PublicAccountUser() {