Update bitrate adjustment with true buffer used size.

This used to be updated with the reserved capacity of the buffer,
not the actual portion in use.

BUG=webrtc:6034

Review-Url: https://codereview.webrtc.org/2620653005
Cr-Commit-Position: refs/heads/master@{#15982}

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()