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

ios/chrome/browser/signin/signin_manager_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/signin_manager_factory.h"
 
 #include <utility>
 
 #include "base/memory/singleton.h"
 #include "base/time/time.h"
@@ skipping 75 matching lines @@
   std::unique_ptr<SigninManager> service(new SigninManager(
       SigninClientFactory::GetForBrowserState(chrome_browser_state),
       OAuth2TokenServiceFactory::GetForBrowserState(chrome_browser_state),
       ios::AccountTrackerServiceFactory::GetForBrowserState(
           chrome_browser_state),
       ios::GaiaCookieManagerServiceFactory::GetForBrowserState(
           chrome_browser_state)));
   service->Initialize(GetApplicationContext()->GetLocalState());
   for (auto& observer : observer_list_)
     observer.SigninManagerCreated(service.get());
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(service);
 }
 
 void SigninManagerFactory::BrowserStateShutdown(web::BrowserState* context) {
   SigninManager* manager =
       static_cast<SigninManager*>(GetServiceForBrowserState(context, false));
   if (manager) {
     for (auto& observer : observer_list_)
       observer.SigninManagerShutdown(manager);
   }
   BrowserStateKeyedServiceFactory::BrowserStateShutdown(context);
 }
 
 }  // namespace ios