Introduce Identity Service and its initial usage

components/signin/public/interfaces/account_id_traits.h

 // Copyright 2017 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_PUBLIC_INTERFACES_ACCOUNT_ID_TRAITS_H_
 #define COMPONENTS_SIGNIN_PUBLIC_INTERFACES_ACCOUNT_ID_TRAITS_H_
 
 #include <string>
 
 #include "components/signin/core/account_id/account_id.h"
@@ skipping 82 matching lines @@
         // Bail if there is no user email.
         if (user_email.empty())
           return false;
 
         *out = AccountId::FromUserEmail(user_email);
         break;
     }
 
     return out->is_valid();
   }
+
+  static bool IsNull(const AccountId& input) { return !input.is_valid(); }
+
+  static void SetToNull(AccountId* output) { *output = EmptyAccountId(); }
 };
 
 }  // namespace mojo
 
 #endif  // COMPONENTS_SIGNIN_PUBLIC_INTERFACES_ACCOUNT_ID_TRAITS_H_