Reland of This CL replaces e-mail with AccountId on user selection screen.

components/signin/core/account_id/account_id.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_SIGNIN_CORE_ACCOUNT_ID_ACCOUNT_ID_H_
 #define COMPONENTS_SIGNIN_CORE_ACCOUNT_ID_ACCOUNT_ID_H_
 
 #include <string>
 #include "base/containers/hash_tables.h"
 
@@ skipping 27 matching lines @@
 
   // This method is to be used during transition period only.
   static AccountId FromUserEmail(const std::string& user_email);
   // This method is to be used during transition period only.
   static AccountId FromGaiaId(const std::string& gaia_id);
   // This method is the preferred way to construct AccountId if you have
   // full account information.
   static AccountId FromUserEmailGaiaId(const std::string& user_email,
                                        const std::string& gaia_id);
 
-  // std::string Serialize() const;
-  // static AccountId Deserialize(const std::string& serialized);
+  // These are (for now) unstable and cannot be used to store serialized data to
+  // persistent storage. Only in-memory storage is safe.
+  // Serialize() returns JSON dictionary,
+  // Deserialize() restores AccountId after serialization.
+  std::string Serialize() const;
+  static bool Deserialize(const std::string& serialized,
+                          AccountId* out_account_id);
 
  private:
   AccountId();
   AccountId(const std::string& gaia_id, const std::string& user_email);
 
   std::string gaia_id_;
   std::string user_email_;
 };
 
 // Returns a reference to a singleton.
 const AccountId& EmptyAccountId();
 
 namespace BASE_HASH_NAMESPACE {
 
 // Implement hashing of AccountId, so it can be used as a key in STL containers.
 template <>
 struct hash<AccountId> {
   std::size_t operator()(const AccountId& user_id) const;
 };
 
 }  // namespace BASE_HASH_NAMESPACE
 
 #endif  // COMPONENTS_SIGNIN_CORE_ACCOUNT_ID_ACCOUNT_ID_H_