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

ios/chrome/browser/ios_chrome_io_thread.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/ios_chrome_io_thread.h"
 
 #include <stddef.h>
 
 #include <utility>
 #include <vector>
@@ skipping 108 matching lines @@
   // If hostname remappings were specified on the command-line, layer these
   // rules on top of the real host resolver. This allows forwarding all requests
   // through a designated test server.
   if (!command_line.HasSwitch(switches::kIOSHostResolverRules))
     return global_host_resolver;
 
   std::unique_ptr<net::MappedHostResolver> remapped_resolver(
       new net::MappedHostResolver(std::move(global_host_resolver)));
   remapped_resolver->SetRulesFromString(
       command_line.GetSwitchValueASCII(switches::kIOSHostResolverRules));
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(remapped_resolver);
 }
 
 int GetSwitchValueAsInt(const base::CommandLine& command_line,
                         const std::string& switch_name) {
   int value;
   if (!base::StringToInt(command_line.GetSwitchValueASCII(switch_name),
                          &value)) {
     return 0;
   }
@@ skipping 428 matching lines @@
 
   globals->system_http_network_session.reset(
       new net::HttpNetworkSession(system_params));
   globals->system_http_transaction_factory.reset(
       new net::HttpNetworkLayer(globals->system_http_network_session.get()));
   context->set_http_transaction_factory(
       globals->system_http_transaction_factory.get());
 
   return context;
 }