ChromeOS cryptohome should be able to use gaia id as user identifier.

components/user_manager/known_user.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_KNOWN_USER_H_
 #define COMPONENTS_USER_MANAGER_KNOWN_USER_H_
 
 #include <string>
+#include <vector>
 
 #include "components/user_manager/user_manager_export.h"
 
 class AccountId;
 class PrefRegistrySimple;
 
 namespace base {
 class DictionaryValue;
 class ListValue;
 class TaskRunner;
@@ skipping 42 matching lines @@
 // fills in |out_value|. Otherwise returns false.
 bool USER_MANAGER_EXPORT GetIntegerPref(const AccountId& account_id,
                                         const std::string& path,
                                         int* out_value);
 
 // Updates user's identified by |account_id| integer preference |path|.
 void USER_MANAGER_EXPORT SetIntegerPref(const AccountId& account_id,
                                         const std::string& path,
                                         const int in_value);
 
+// Returns the list of known AccountIds.
+std::vector<AccountId> USER_MANAGER_EXPORT GetKnownAccountIds();
+
 // This call forms full account id of a known user by email and (optionally)
 // gaia_id.
 // This is a temporary call while migrating to AccountId.
 AccountId USER_MANAGER_EXPORT GetAccountId(const std::string& user_email,
                                            const std::string& gaia_id);
 
 // Returns true if |subsystem| data was migrated to GaiaId for the |account_id|.
 bool USER_MANAGER_EXPORT GetGaiaIdMigrationStatus(const AccountId& account_id,
                                                   const std::string& subsystem);
 
@@ skipping 51 matching lines @@
 // Removes all user preferences associated with |account_id|.
 // (This one used by user_manager only and thus not exported.)
 void RemovePrefs(const AccountId& account_id);
 
 // Register known user prefs.
 void USER_MANAGER_EXPORT RegisterPrefs(PrefRegistrySimple* registry);
 }
 }  // namespace user_manager
 
 #endif  // COMPONENTS_USER_MANAGER_KNOWN_USER_H_