Make the bitrate_allocator param optional to prepare for its removal

in https://webrtc-review.googlesource.com/109040

Bug: webrtc:9513
Change-Id: I676e5e0242f068b12764a52bf8b6a6865ea7f120
Reviewed-on: https://webrtc-review.googlesource.com/c/110142
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25558}
diff --git a/modules/video_coding/include/video_codec_initializer.h b/modules/video_coding/include/video_codec_initializer.h
index ee70810..6da5398 100644
--- a/modules/video_coding/include/video_codec_initializer.h
+++ b/modules/video_coding/include/video_codec_initializer.h
@@ -34,7 +34,7 @@
       const VideoEncoderConfig& config,
       const std::vector<VideoStream>& streams,
       VideoCodec* codec,
-      std::unique_ptr<VideoBitrateAllocator>* bitrate_allocator);
+      std::unique_ptr<VideoBitrateAllocator>* bitrate_allocator = nullptr);
 
   // Create a bitrate allocator for the specified codec.
   static std::unique_ptr<VideoBitrateAllocator> CreateBitrateAllocator(