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

ios/chrome/browser/bookmarks/bookmark_model_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/bookmarks/bookmark_model_factory.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "base/memory/singleton.h"
@@ skipping 55 matching lines @@
       new bookmarks::BookmarkModel(
           base::MakeUnique<BookmarkClientImpl>(browser_state)));
   bookmark_model->Load(
       browser_state->GetPrefs(),
       browser_state->GetStatePath(),
       ios::StartupTaskRunnerServiceFactory::GetForBrowserState(browser_state)
           ->GetBookmarkTaskRunner(),
       web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
   ios::BookmarkUndoServiceFactory::GetForBrowserState(browser_state)
       ->Start(bookmark_model.get());
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(bookmark_model);
 }
 
 web::BrowserState* BookmarkModelFactory::GetBrowserStateToUse(
     web::BrowserState* context) const {
   return GetBrowserStateRedirectedInIncognito(context);
 }
 
 bool BookmarkModelFactory::ServiceIsNULLWhileTesting() const {
   return true;
 }
 
 }  // namespace ios