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

components/user_manager/user_id.cc

Index: components/user_manager/user_id.cc
diff --git a/components/user_manager/user_id.cc b/components/user_manager/user_id.cc
new file mode 100644
index 0000000000000000000000000000000000000000..9f80c70826e340ee0e1c14b242696a68f642abb2
--- /dev/null
+++ b/components/user_manager/user_id.cc
@@ -0,0 +1,103 @@
+// 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.
+
+#include "components/user_manager/user_id.h"
+
+#include "base/memory/singleton.h"
+#include "google_apis/gaia/gaia_auth_util.h"
+
+namespace {
+
+struct EmptyUserID {
+  EmptyUserID() : user_id(std::string(), std::string()) {}
+  const user_manager::UserID user_id;
+
+  static EmptyUserID* GetInstance() {
+    return Singleton<EmptyUserID>::get();
+  }
+};
+
+} // anonymous namespace
+
+namespace user_manager {
+
+UserID::UserID(const std::string& gaia_id, const std::string& user_email) : gaia_id_(gaia_id), user_email_(user_email) {}
+
+UserID::UserID(const UserID& other) : gaia_id_(other.gaia_id_), user_email_(other.user_email_) {}
+
+bool UserID::operator==(const UserID& other) const {
+  return (!gaia_id_.empty() && gaia_id_ == other.gaia_id_) || (!user_email_.empty() && user_email_ == other.user_email_);

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:44]

what if our gaia_id_ is not empty, but other.gaia_id_ is?
Same for user email?

+}
+
+bool UserID::operator!=(const UserID& other) const {
+  return (!gaia_id_.empty() && gaia_id_ == other.gaia_id_) || (!user_email_.empty() && user_email_ == other.user_email_);
+}
+
+bool UserID::operator<(const UserID& right) const {
+  return user_email_ < right.user_email_;

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:44]

What if e-mails are both empty, but GaiaIDs are different?
We should be extremely careful with <, == and Hash contracts, as they are used
in data structures.

+}
+
+bool UserID::empty() const {
+  return gaia_id_.empty() && user_email_.empty();
+}
+
+void UserID::clear() {
+  gaia_id_.clear();
+  user_email_.clear();
+}
+
+const std::string& UserID::GetGaiaId() const {
+  return gaia_id_;
+}
+
+const std::string& UserID::GetUserEmail() const {
+  return user_email_;
+}
+
+void UserID::SetGaiaId(const std::string& gaia_id) {
+  gaia_id_ = gaia_id;
+}
+
+void UserID::SetUserEmail(const std::string& email) {
+  user_email_ = email;
+}
+
+// static
+UserID UserID::FromUserEmail(const std::string& email) {
+  return UserID(std::string() /* gaia_id */, email);
+}
+
+// static
+UserID UserID::Deserialize(const std::string& serialized) {
+  // If the account_id is an email address, then canonicalize it.
+  if (serialized.find('@') != std::string::npos)
+    return UserID(std::string() /* gaia_id */, gaia::CanonicalizeEmail(serialized));
+
+#ifdef OS_CHROME
+  if (serialized == chromeos::login::kLegacyGuestUserName)
+    return UserID(std::string() /* gaia_id */, gaia::CanonicalizeEmail(serialized));
+#endif
+
+  // This will change once UserID will change.
+  return UserID(std::string() /* gaia_id */, serialized);
+}
+
+std::string UserID::Serialize() const {
+  return user_email_;
+}
+
+const UserID& EmptyUserID() {
+  return EmptyUserID::GetInstance()->user_id;
+}
+
+}  // namespace user_manager
+
+namespace BASE_HASH_NAMESPACE {
+
+std::size_t
+hash<user_manager::UserID>::operator()(const user_manager::UserID& user_id) const {
+  return std::hash<std::string>()(user_id.GetUserEmail());
+}
+
+} // namespace BASE_HASH_NAMESPACE