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

ppapi/proxy/ppb_flash_menu_proxy.cc

Index: ppapi/proxy/ppb_flash_menu_proxy.cc
===================================================================
--- ppapi/proxy/ppb_flash_menu_proxy.cc	(revision 94913)
+++ ppapi/proxy/ppb_flash_menu_proxy.cc	(working copy)
@@ -162,22 +162,14 @@
                                      const PP_Point& location) {
   ShowRequest* request = new ShowRequest;
   request->menu = menu;
-  CompletionCallback callback = callback_factory_.NewOptionalCallback(
-      &PPB_Flash_Menu_Proxy::SendShowACKToPlugin, request);
 
-  EnterHostFromHostResource<PPB_Flash_Menu_API> enter(menu);
-  int32_t result = PP_ERROR_BADRESOURCE;
+  EnterHostFromHostResourceForceCallback<PPB_Flash_Menu_API> enter(
+      menu, callback_factory_, &PPB_Flash_Menu_Proxy::SendShowACKToPlugin,
+      request);
   if (enter.succeeded()) {
-    result = enter.object()->Show(&location,
-                                  &request->selected_id,
-                                  callback.pp_completion_callback());
+    enter.SetResult(enter.object()->Show(&location, &request->selected_id,
+                                         enter.callback()));
   }
-  if (result != PP_OK_COMPLETIONPENDING) {
-    // There was some error, so we won't get a callback. We need to now issue
-    // the ACK to the plugin so that it hears about the error. This will also
-    // clean up the data associated with the callback.
-    callback.Run(result);
-  }
 }
 
 void PPB_Flash_Menu_Proxy::OnMsgShowACK(const HostResource& menu,