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

ios/chrome/browser/browser_state/test_chrome_browser_state.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.
 
 #include "ios/chrome/browser/browser_state/test_chrome_browser_state.h"
 
 #include <utility>
 
 #include "base/base_paths.h"
 #include "base/files/file_util.h"
@@ skipping 54 matching lines @@
 std::unique_ptr<KeyedService> BuildBookmarkModel(web::BrowserState* context) {
   ios::ChromeBrowserState* browser_state =
       ios::ChromeBrowserState::FromBrowserState(context);
   std::unique_ptr<bookmarks::BookmarkModel> bookmark_model(
       new bookmarks::BookmarkModel(
           base::MakeUnique<BookmarkClientImpl>(browser_state)));
   bookmark_model->Load(
       browser_state->GetPrefs(),
       browser_state->GetStatePath(), browser_state->GetIOTaskRunner(),
       web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(bookmark_model);
 }
 
 void NotReachedErrorCallback(WebDataServiceWrapper::ErrorType,
                              sql::InitStatus,
                              const std::string&) {
   NOTREACHED();
 }
 
 std::unique_ptr<KeyedService> BuildWebDataService(web::BrowserState* context) {
@@ skipping 331 matching lines @@
   pref_service_ = std::move(prefs);
 }
 
 std::unique_ptr<TestChromeBrowserState>
 TestChromeBrowserState::Builder::Build() {
   DCHECK(!build_called_);
   return base::WrapUnique(new TestChromeBrowserState(
       state_path_, std::move(pref_service_), testing_factories_,
       refcounted_testing_factories_));
 }