Pass CompletionCallbacks by reference

ppapi/cpp/dev/video_decoder_dev.cc

Index: ppapi/cpp/dev/video_decoder_dev.cc
diff --git a/ppapi/cpp/dev/video_decoder_dev.cc b/ppapi/cpp/dev/video_decoder_dev.cc
index 92bbe1c6352594d7beb1373b467ac87da3481bb4..ee03a55e889e67b8ef9af571cf4051f234e61f3d 100644
--- a/ppapi/cpp/dev/video_decoder_dev.cc
+++ b/ppapi/cpp/dev/video_decoder_dev.cc
@@ -48,7 +48,7 @@ void VideoDecoder_Dev::AssignPictureBuffers(
 
 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(
@@ -62,7 +62,7 @@ void VideoDecoder_Dev::ReusePictureBuffer(int32_t picture_buffer_id) {
       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(