Convert the async device ID getter to a chrome resource host

ppapi/host/host_message_context.cc

 // Copyright (c) 2012 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 "ppapi/host/host_message_context.h"
 
 namespace ppapi {
 namespace host {
 
 HostMessageContext::HostMessageContext(
     const ppapi::proxy::ResourceMessageCallParams& cp)
     : params(cp) {
 }
 
 HostMessageContext::~HostMessageContext() {
 }
 
-ppapi::proxy::ResourceMessageReplyParams HostMessageContext::MakeReplyParams() {
+ppapi::proxy::ResourceMessageReplyParams
+HostMessageContext::MakeReplyParams() const {
   return ppapi::proxy::ResourceMessageReplyParams(params.pp_resource(),
                                                   params.sequence());
 }
 
 }  // namespace host
 }  // namespace ppapi