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

chromeos/cryptohome/cryptohome_parameters.cc

Index: chromeos/cryptohome/cryptohome_parameters.cc
diff --git a/chromeos/cryptohome/cryptohome_parameters.cc b/chromeos/cryptohome/cryptohome_parameters.cc
index 8b65a710e471f53b8ba07dbf8719901bd53b1643..a36770416ead59c61c07a9d0be6e2beb1135090c 100644
--- a/chromeos/cryptohome/cryptohome_parameters.cc
+++ b/chromeos/cryptohome/cryptohome_parameters.cc
@@ -7,15 +7,37 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "chromeos/chromeos_switches.h"
 #include "chromeos/dbus/cryptohome/key.pb.h"
+#include "components/signin/core/account_id/account_id.h"
 
 namespace cryptohome {
+namespace {
 
-Identification::Identification(const std::string& user_id) : user_id(user_id) {
+const std::string GetCryptohomeId(const AccountId& account_id) {
+  // Guest account has empty GaiaId. Default to email.

[xiyuan, 2016/02/17 23:14:25]

Think this is true all non-Gaia accounts in addition to Guest, e.g. kiosk apps,
public session etc.

[Alexander Alekseev, 2016/02/18 13:45:14]

Done.

+  if (account_id.GetGaiaId().empty())
+    return account_id.GetUserEmail();  // Migrated
+
+  const bool is_gaiaid_migration_started =
+      chromeos::switches::IsGaiaIdMigrationStarted();
+  return is_gaiaid_migration_started ? account_id.GetGaiaIdKey()
+                                     : account_id.GetUserEmail();  // Migrated
+}
+
+}  //  anonymous namespace
+
+Identification::Identification(const AccountId& account_id)
+    : id_(GetCryptohomeId(account_id)) {}
+
+Identification::Identification(const std::string& id) : id_(id) {}
+
+Identification Identification::FromString(const std::string& id) {
+  return Identification(id);
 }
 
 bool Identification::operator==(const Identification& other) const {
-  return user_id == other.user_id;
+  return id_ == other.id_;
 }
 
 KeyDefinition::AuthorizationData::Secret::Secret() : encrypt(false),