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

components/user_manager/user_id.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_USER_MANAGER_USER_ID_H_
 #define COMPONENTS_USER_MANAGER_USER_ID_H_
 
 #include <string>
+#include "base/containers/hash_tables.h"
 
 namespace user_manager {
 
 // Type that contains enough information to identify user on ChromeOS.
-typedef std::string UserID;
+//typedef std::string UserID;
+
+class UserID {
+ public:
+  UserID(const std::string& gaia_id, const std::string& user_email);

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:45]

I would make this one private and force using FromEMail / FromGaiaID / FromBoth.

+
+  UserID(const UserID& other);
+
+  bool operator==(const UserID& other) const;
+  bool operator!=(const UserID& other) const;
+  bool operator<(const UserID& right) const;
+
+  bool empty() const;
+  void clear();
+
+  const std::string& GetGaiaId() const;
+  const std::string& GetUserEmail() const;
+
+  void SetGaiaId(const std::string& gaia_id);
+  void SetUserEmail(const std::string& email);
+
+  std::string Serialize() const;
+
+  static UserID FromUserEmail(const std::string& user_email);
+  static UserID Deserialize(const std::string& serialized);
+
+ private:
+  std::string gaia_id_;
+  std::string user_email_;
+};
+
+// Returns a reference to a singleton.
+const UserID& EmptyUserID();
 
 }  // namespace user_manager
 
+namespace BASE_HASH_NAMESPACE {
+
+// Implement hashing of UserID, so it can be used as a key in STL containers.
+template<>
+struct hash<user_manager::UserID> {
+  std::size_t operator()(const user_manager::UserID& user_id) const;
+};
+
+}  // namespace BASE_HASH_NAMESPACE
+
 #endif  // COMPONENTS_USER_MANAGER_USER_ID_H_