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

components/user_manager/known_user.h

Index: components/user_manager/known_user.h
diff --git a/components/user_manager/known_user.h b/components/user_manager/known_user.h
index aa02b2fc6373bcf355b81fd9c0b78b6eab3b6a9f..8a16cbe146f572b62822b22d8ee92be470827e43 100644
--- a/components/user_manager/known_user.h
+++ b/components/user_manager/known_user.h
@@ -6,6 +6,7 @@
 #define COMPONENTS_USER_MANAGER_KNOWN_USER_H_
 
 #include <string>
+#include <vector>
 
 #include "components/user_manager/user_manager_export.h"
 
@@ -68,6 +69,9 @@ 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.