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

ios/chrome/browser/signin/signin_client_impl.mm

 // 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_client_impl.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 126 matching lines @@
 std::unique_ptr<SigninClient::CookieChangedSubscription>
 SigninClientImpl::AddCookieChangedCallback(
     const GURL& url,
     const std::string& name,
     const net::CookieStore::CookieChangedCallback& callback) {
   scoped_refptr<net::URLRequestContextGetter> context_getter =
       browser_state_->GetRequestContext();
   DCHECK(context_getter.get());
   std::unique_ptr<SigninCookieChangedSubscription> subscription(
       new SigninCookieChangedSubscription(context_getter, url, name, callback));
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(subscription);
 }
 
 void SigninClientImpl::OnSignedIn(const std::string& account_id,
                                   const std::string& gaia_id,
                                   const std::string& username,
                                   const std::string& password) {
   ios::ChromeBrowserStateManager* browser_state_manager =
       GetApplicationContext()->GetChromeBrowserStateManager();
   BrowserStateInfoCache* cache =
@@ skipping 75 matching lines @@
   }
 }
 
 std::unique_ptr<GaiaAuthFetcher> SigninClientImpl::CreateGaiaAuthFetcher(
     GaiaAuthConsumer* consumer,
     const std::string& source,
     net::URLRequestContextGetter* getter) {
   return base::MakeUnique<GaiaAuthFetcherIOS>(consumer, source, getter,
                                               browser_state_);
 }