Pass CompletionCallbacks by reference

ppapi/cpp/dev/video_decoder_dev.h

Index: ppapi/cpp/dev/video_decoder_dev.h
diff --git a/ppapi/cpp/dev/video_decoder_dev.h b/ppapi/cpp/dev/video_decoder_dev.h
index c33ce2d3d9e8830d70f57f31ce5aba593e87603e..e9242203db9eb688fb8f4af8ebf528c75a450f79 100644
--- a/ppapi/cpp/dev/video_decoder_dev.h
+++ b/ppapi/cpp/dev/video_decoder_dev.h
@@ -34,9 +34,9 @@ class VideoDecoder_Dev : public Resource {
   // PPB_VideoDecoder_Dev implementation.
   void AssignPictureBuffers(const std::vector<PP_PictureBuffer_Dev>& buffers);
   int32_t Decode(const PP_VideoBitstreamBuffer_Dev& bitstream_buffer,
-                 CompletionCallback callback);
+                 const CompletionCallback& callback);
   void ReusePictureBuffer(int32_t picture_buffer_id);
-  int32_t Flush(CompletionCallback callback);
+  int32_t Flush(const CompletionCallback& callback);
   int32_t Reset(const CompletionCallback& callback);
 
  private: