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

ios/chrome/browser/history/history_service_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/history/history_service_factory.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "base/memory/singleton.h"
@@ skipping 63 matching lines @@
       ios::ChromeBrowserState::FromBrowserState(context);
   std::unique_ptr<history::HistoryService> history_service(
       new history::HistoryService(
           base::MakeUnique<HistoryClientImpl>(
               ios::BookmarkModelFactory::GetForBrowserState(browser_state)),
           nullptr));
   if (!history_service->Init(history::HistoryDatabaseParamsForPath(
           browser_state->GetStatePath()))) {
     return nullptr;
   }
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(history_service);
 }
 
 web::BrowserState* HistoryServiceFactory::GetBrowserStateToUse(
     web::BrowserState* context) const {
   return GetBrowserStateRedirectedInIncognito(context);
 }
 
 bool HistoryServiceFactory::ServiceIsNULLWhileTesting() const {
   return true;
 }
 
 }  // namespace ios