Update bitrate adjustment with true buffer used size.

webrtc/sdk/objc/Framework/Classes/h264_video_toolbox_encoder.mm

Index: webrtc/sdk/objc/Framework/Classes/h264_video_toolbox_encoder.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/h264_video_toolbox_encoder.mm b/webrtc/sdk/objc/Framework/Classes/h264_video_toolbox_encoder.mm
index ea4cd04f2203c066c4c4788d9c5c9edca76663d2..e50b225f2d3f44dbf62ad9e79e418f761a00475b 100644
--- a/webrtc/sdk/objc/Framework/Classes/h264_video_toolbox_encoder.mm
+++ b/webrtc/sdk/objc/Framework/Classes/h264_video_toolbox_encoder.mm
@@ -731,7 +731,7 @@ void H264VideoToolboxEncoder::OnEncodedFrame(
     LOG(LS_ERROR) << "Encode callback failed: " << res.error;
     return;
   }
-  bitrate_adjuster_.Update(frame._size);
+  bitrate_adjuster_.Update(frame._length);
 }
 
 VideoEncoder::ScalingSettings H264VideoToolboxEncoder::GetScalingSettings()