Pass CompletionCallbacks by reference

ppapi/cpp/dev/video_decoder_dev.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/cpp/dev/video_decoder_dev.h"
 
 #include "ppapi/c/dev/ppb_video_decoder_dev.h"
 #include "ppapi/c/dev/ppp_video_decoder_dev.h"
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/cpp/graphics_3d.h"
@@ skipping 30 matching lines @@
 void VideoDecoder_Dev::AssignPictureBuffers(
     const std::vector<PP_PictureBuffer_Dev>& buffers) {
   if (!has_interface<PPB_VideoDecoder_Dev>() || !pp_resource())
     return;
   get_interface<PPB_VideoDecoder_Dev>()->AssignPictureBuffers(
       pp_resource(), buffers.size(), &buffers[0]);
 }
 
 int32_t VideoDecoder_Dev::Decode(
     const PP_VideoBitstreamBuffer_Dev& bitstream_buffer,
-    CompletionCallback callback) {
+    const CompletionCallback& callback) {
   if (!has_interface<PPB_VideoDecoder_Dev>())
     return callback.MayForce(PP_ERROR_NOINTERFACE);
   return get_interface<PPB_VideoDecoder_Dev>()->Decode(
       pp_resource(), &bitstream_buffer, callback.pp_completion_callback());
 }
 
 void VideoDecoder_Dev::ReusePictureBuffer(int32_t picture_buffer_id) {
   if (!has_interface<PPB_VideoDecoder_Dev>() || !pp_resource())
     return;
   get_interface<PPB_VideoDecoder_Dev>()->ReusePictureBuffer(
       pp_resource(), picture_buffer_id);
 }
 
-int32_t VideoDecoder_Dev::Flush(CompletionCallback callback) {
+int32_t VideoDecoder_Dev::Flush(const CompletionCallback& callback) {
   if (!has_interface<PPB_VideoDecoder_Dev>())
     return callback.MayForce(PP_ERROR_NOINTERFACE);
   return get_interface<PPB_VideoDecoder_Dev>()->Flush(
       pp_resource(), callback.pp_completion_callback());
 }
 
 int32_t VideoDecoder_Dev::Reset(const CompletionCallback& callback) {
   if (!has_interface<PPB_VideoDecoder_Dev>())
     return callback.MayForce(PP_ERROR_NOINTERFACE);
   return get_interface<PPB_VideoDecoder_Dev>()->Reset(
       pp_resource(), callback.pp_completion_callback());
 }
 
 }  // namespace pp