Introduce Identity Service and its initial usage

components/signin/public/interfaces/account_id_traits.h

Index: components/signin/public/interfaces/account_id_traits.h
diff --git a/components/signin/public/interfaces/account_id_traits.h b/components/signin/public/interfaces/account_id_traits.h
index cc1d39de91773289c8fe34eaeb1a56d05b334390..a2107cbb30ba76df453df58e7313a8f1f023478b 100644
--- a/components/signin/public/interfaces/account_id_traits.h
+++ b/components/signin/public/interfaces/account_id_traits.h
@@ -100,6 +100,10 @@ struct StructTraits<signin::mojom::AccountIdDataView, AccountId> {
 
     return out->is_valid();
   }
+
+  static bool IsNull(const AccountId& input) { return !input.is_valid(); }
+
+  static void SetToNull(AccountId* output) { *output = EmptyAccountId(); }
 };
 
 }  // namespace mojo