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

components/user_manager/fake_user_manager.cc

Index: components/user_manager/fake_user_manager.cc
diff --git a/components/user_manager/fake_user_manager.cc b/components/user_manager/fake_user_manager.cc
index 5e8d5613e3c1e8b555e93e078025fd6bf1542b97..5a3ba8b3ad9b9439a02824fcae1f9d45f0c12e81 100644
--- a/components/user_manager/fake_user_manager.cc
+++ b/components/user_manager/fake_user_manager.cc
@@ -32,21 +32,21 @@ namespace user_manager {
 FakeUserManager::FakeUserManager()
     : UserManagerBase(new FakeTaskRunner(), new FakeTaskRunner()),
       primary_user_(NULL),
-      owner_email_(std::string()) {
+      owner_id_(UserID(std::string() /* gaia_id */, std::string() /* user_email */)) {
 }
 
 FakeUserManager::~FakeUserManager() {
 }
 
-const user_manager::User* FakeUserManager::AddUser(const std::string& email) {
-  user_manager::User* user = user_manager::User::CreateRegularUser(email);
+const user_manager::User* FakeUserManager::AddUser(const user_manager::UserID& user_id) {
+  user_manager::User* user = user_manager::User::CreateRegularUser(user_id);
   users_.push_back(user);
   return user;
 }
 
-void FakeUserManager::RemoveUserFromList(const std::string& email) {
+void FakeUserManager::RemoveUserFromList(const UserID& user_id) {
   user_manager::UserList::iterator it = users_.begin();
-  while (it != users_.end() && (*it)->email() != email)
+  while (it != users_.end() && (*it)->GetUserID() != user_id)
     ++it;
   if (it != users_.end()) {
     delete *it;
@@ -73,7 +73,7 @@ const user_manager::UserList& FakeUserManager::GetLoggedInUsers() const {
   return logged_in_users_;
 }
 
-void FakeUserManager::UserLoggedIn(const std::string& email,
+void FakeUserManager::UserLoggedIn(const UserId& user_id,
                                    const std::string& username_hash,
                                    bool browser_restart) {
   for (user_manager::UserList::const_iterator it = users_.begin();
@@ -95,7 +95,7 @@ user_manager::User* FakeUserManager::GetActiveUserInternal() const {
     if (!active_user_id_.empty()) {
       for (user_manager::UserList::const_iterator it = users_.begin();
            it != users_.end(); ++it) {
-        if ((*it)->email() == active_user_id_)
+        if ((*it)->GetUserID() == active_user_id_)
           return *it;
       }
     }
@@ -112,14 +112,14 @@ user_manager::User* FakeUserManager::GetActiveUser() {
   return GetActiveUserInternal();
 }
 
-void FakeUserManager::SwitchActiveUser(const std::string& email) {
+void FakeUserManager::SwitchActiveUser(const UserID& user_id) {
 }
 
-void FakeUserManager::SaveUserDisplayName(const std::string& username,
+void FakeUserManager::SaveUserDisplayName(const UserID& user_id,
                                           const base::string16& display_name) {
   for (user_manager::UserList::iterator it = users_.begin(); it != users_.end();
        ++it) {
-    if ((*it)->email() == username) {
+    if ((*it)->GetUserID() == user_id) {
       (*it)->set_display_name(display_name);
       return;
     }
@@ -134,27 +134,27 @@ user_manager::UserList FakeUserManager::GetUnlockUsers() const {
   return users_;
 }
 
-const std::string& FakeUserManager::GetOwnerEmail() const {
-  return owner_email_;
+const user_manager::UserID& FakeUserManager::GetOwnerID() const {
+  return owner_id_;
 }
 
-bool FakeUserManager::IsKnownUser(const std::string& email) const {
+bool FakeUserManager::IsKnownUser(const UserID& user_id) const {
   return true;
 }
 
 const user_manager::User* FakeUserManager::FindUser(
-    const std::string& email) const {
+    const UserID& user_id) const {
   const user_manager::UserList& users = GetUsers();
   for (user_manager::UserList::const_iterator it = users.begin();
        it != users.end(); ++it) {
-    if ((*it)->email() == email)
+    if ((*it)->GetUserID() == user_id)
       return *it;
   }
   return NULL;
 }
 
 user_manager::User* FakeUserManager::FindUserAndModify(
-    const std::string& email) {
+    const const UserID& user_id) {
   return NULL;
 }
 
@@ -171,12 +171,12 @@ const user_manager::User* FakeUserManager::GetPrimaryUser() const {
 }
 
 base::string16 FakeUserManager::GetUserDisplayName(
-    const std::string& username) const {
+    const UserID& user_id) const {
   return base::string16();
 }
 
 std::string FakeUserManager::GetUserDisplayEmail(
-    const std::string& username) const {
+    const user_manager::UserID& user_id) const {
   return std::string();
 }
 
@@ -232,7 +232,7 @@ bool FakeUserManager::IsSessionStarted() const {
 }
 
 bool FakeUserManager::IsUserNonCryptohomeDataEphemeral(
-    const std::string& email) const {
+    const UserID& user_id) const {
   return false;
 }
 
@@ -257,16 +257,16 @@ bool FakeUserManager::IsEnterpriseManaged() const {
   return false;
 }
 
-bool FakeUserManager::IsDemoApp(const std::string& user_id) const {
+bool FakeUserManager::IsDemoApp(const UserID& user_id) const {
   return false;
 }
 
-bool FakeUserManager::IsKioskApp(const std::string& user_id) const {
+bool FakeUserManager::IsKioskApp(const UserID& user_id) const {
   return false;
 }
 
 bool FakeUserManager::IsPublicAccountMarkedForRemoval(
-    const std::string& user_id) const {
+    const UserID& user_id) const {
   return false;
 }