Add a template to handle properly issuing completion callbacks. This fixes

ppapi/proxy/ppb_file_chooser_proxy.cc

 // Copyright (c) 2011 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/proxy/ppb_file_chooser_proxy.h"
 
 #include <queue>
 
 #include "ppapi/c/dev/ppb_file_chooser_dev.h"
 #include "ppapi/c/pp_errors.h"
@@ skipping 176 matching lines @@
                                         const std::string& accept_mime_types,
                                         HostResource* result) {
   PP_FileChooserOptions_Dev options;
   options.mode = static_cast<PP_FileChooserMode_Dev>(mode);
   options.accept_mime_types = accept_mime_types.c_str();
   result->SetHostResource(
       instance, ppb_file_chooser_target()->Create(instance, &options));
 }
 
 void PPB_FileChooser_Proxy::OnMsgShow(const HostResource& chooser) {
-  CompletionCallback callback = callback_factory_.NewOptionalCallback(
-      &PPB_FileChooser_Proxy::OnShowCallback, chooser);
-
-  int32_t result = ppb_file_chooser_target()->Show(
-      chooser.host_resource(), callback.pp_completion_callback());
-  if (result != PP_OK_COMPLETIONPENDING)
-    callback.Run(result);
+  EnterHostFromHostResourceForceCallback<PPB_FileChooser_API> enter(
+      chooser, callback_factory_, &PPB_FileChooser_Proxy::OnShowCallback,
+      chooser);
+  if (enter.succeeded())
+    enter.SetResult(enter.object()->Show(enter.callback()));
 }
 
 void PPB_FileChooser_Proxy::OnMsgChooseComplete(
     const HostResource& chooser,
     int32_t result_code,
     const std::vector<PPBFileRef_CreateInfo>& chosen_files) {
   EnterPluginFromHostResource<PPB_FileChooser_API> enter(chooser);
   if (enter.succeeded()) {
     static_cast<FileChooser*>(enter.object())->ChooseComplete(
         result_code, chosen_files);
@@ skipping 20 matching lines @@
       files.push_back(cur_create_info);
     }
   }
 
   dispatcher()->Send(new PpapiMsg_PPBFileChooser_ChooseComplete(
       INTERFACE_ID_PPB_FILE_CHOOSER, chooser, result, files));
 }
 
 }  // namespace proxy
 }  // namespace pp