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

ppapi/proxy/ppb_graphics_2d_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_graphics_2d_proxy.h"
 
 #include <string.h>  // For memset.
 
 #include "base/compiler_specific.h"
 #include "base/logging.h"
@@ skipping 222 matching lines @@
 void PPB_Graphics2D_Proxy::OnMsgReplaceContents(
     const HostResource& graphics_2d,
     const HostResource& image_data) {
   EnterHostFromHostResource<PPB_Graphics2D_API> enter(graphics_2d);
   if (enter.failed())
     return;
   enter.object()->ReplaceContents(image_data.host_resource());
 }
 
 void PPB_Graphics2D_Proxy::OnMsgFlush(const HostResource& graphics_2d) {
-  CompletionCallback callback = callback_factory_.NewOptionalCallback(
+  EnterHostFromHostResourceForceCallback<PPB_Graphics2D_API> enter(
+      graphics_2d, callback_factory_,
       &PPB_Graphics2D_Proxy::SendFlushACKToPlugin, graphics_2d);
-  int32_t result = ppb_graphics_2d_target()->Flush(
-      graphics_2d.host_resource(), callback.pp_completion_callback());
-  if (result != PP_OK_COMPLETIONPENDING) {
-    // There was some error, so we won't get a flush callback. We need to now
-    // issue the ACK to the plugin hears about the error. This will also clean
-    // up the data associated with the callback.
-    callback.Run(result);
-  }
+  if (enter.failed())
+    return;
+  enter.SetResult(enter.object()->Flush(enter.callback()));
 }
 
 void PPB_Graphics2D_Proxy::OnMsgFlushACK(const HostResource& host_resource,
                                          int32_t pp_error) {
   EnterPluginFromHostResource<PPB_Graphics2D_API> enter(host_resource);
   if (enter.succeeded())
     static_cast<Graphics2D*>(enter.object())->FlushACK(pp_error);
 }
 
 void PPB_Graphics2D_Proxy::SendFlushACKToPlugin(
     int32_t result,
     const HostResource& graphics_2d) {
   dispatcher()->Send(new PpapiMsg_PPBGraphics2D_FlushACK(
       INTERFACE_ID_PPB_GRAPHICS_2D, graphics_2d, result));
 }
 
 }  // namespace proxy
 }  // namespace pp