[ios] Add TODO to record usages of "return std::move(foo)" pattern.

ios/chrome/browser/signin/about_signin_internals_factory.cc

 // 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.
 
 #include "ios/chrome/browser/signin/about_signin_internals_factory.h"
 
 #include <utility>
 
 #include "base/memory/singleton.h"
 #include "components/keyed_service/ios/browser_state_dependency_manager.h"
@@ skipping 41 matching lines @@
       ios::ChromeBrowserState::FromBrowserState(context);
   std::unique_ptr<AboutSigninInternals> service(new AboutSigninInternals(
       OAuth2TokenServiceFactory::GetForBrowserState(chrome_browser_state),
       AccountTrackerServiceFactory::GetForBrowserState(chrome_browser_state),
       SigninManagerFactory::GetForBrowserState(chrome_browser_state),
       SigninErrorControllerFactory::GetForBrowserState(chrome_browser_state),
       GaiaCookieManagerServiceFactory::GetForBrowserState(
           chrome_browser_state)));
   service->Initialize(
       SigninClientFactory::GetForBrowserState(chrome_browser_state));
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(service);
 }
 
 void AboutSigninInternalsFactory::RegisterBrowserStatePrefs(
     user_prefs::PrefRegistrySyncable* user_prefs) {
   AboutSigninInternals::RegisterPrefs(user_prefs);
 }
 
 }  // namespace ios