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

ios/chrome/browser/signin/authentication_service_factory.mm

 // Copyright 2013 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.
 
 #import "ios/chrome/browser/signin/authentication_service_factory.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/memory/singleton.h"
@@ skipping 40 matching lines @@
 std::unique_ptr<KeyedService>
 AuthenticationServiceFactory::BuildServiceInstanceFor(
     web::BrowserState* context) const {
   ios::ChromeBrowserState* browser_state =
       ios::ChromeBrowserState::FromBrowserState(context);
   std::unique_ptr<AuthenticationService> service(new AuthenticationService(
       browser_state,
       OAuth2TokenServiceFactory::GetForBrowserState(browser_state),
       SyncSetupServiceFactory::GetForBrowserState(browser_state)));
   service->Initialize();
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(service);
 }
 
 void AuthenticationServiceFactory::RegisterBrowserStatePrefs(
     user_prefs::PrefRegistrySyncable* registry) {
   AuthenticationService::RegisterPrefs(registry);
 }