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

components/user_manager/known_user.cc

Index: components/user_manager/known_user.cc
diff --git a/components/user_manager/known_user.cc b/components/user_manager/known_user.cc
index 973e12b78ee890d584ccda602932cc6682207af4..84634fafc6f5f9d5e82e07485e5a4f989a07fa4c 100644
--- a/components/user_manager/known_user.cc
+++ b/components/user_manager/known_user.cc
@@ -256,6 +256,30 @@ AccountId GetAccountId(const std::string& user_email,
               : AccountId::FromUserEmailGaiaId(user_email, gaia_id));
 }
 
+std::vector<AccountId> GetKnownAccountIds() {
+  std::vector<AccountId> result;
+  PrefService* local_state = GetLocalState();
+
+  // Local State may not be initialized in tests.
+  if (!local_state)
+    return result;
+
+  const base::ListValue* known_users = local_state->GetList(kKnownUsers);
+  for (size_t i = 0; i < known_users->GetSize(); ++i) {
+    const base::DictionaryValue* element = nullptr;
+    if (known_users->GetDictionary(i, &element)) {
+      std::string email;
+      std::string gaia_id;
+      const bool has_email = element->GetString(kCanonicalEmail, &email);
+      const bool has_gaia_id = element->GetString(kGAIAIdKey, &gaia_id);
+      if (has_email || has_gaia_id) {
+        result.push_back(AccountId::FromUserEmailGaiaId(email, gaia_id));
+      }
+    }
+  }
+  return result;
+}
+
 bool GetGaiaIdMigrationStatus(const AccountId& account_id,
                               const std::string& subsystem) {
   bool migrated = false;