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

components/signin/core/account_id/account_id.h

Index: components/signin/core/account_id/account_id.h
diff --git a/components/signin/core/account_id/account_id.h b/components/signin/core/account_id/account_id.h
index 73be4501db5b0ab107937acced10ee17d478a5e2..9f72030bd0e73378dd1d16d66b445e12402d927a 100644
--- a/components/signin/core/account_id/account_id.h
+++ b/components/signin/core/account_id/account_id.h
@@ -45,8 +45,13 @@
   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();