AGC: Removing unnneccessary copying and changing to using const

The changes have been shown to be bitexact on a large dataset.

Bug: webrtc:10859
Change-Id: Iedc0e9e944ebfabb717dd7fb4d2682c695da883e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159694
Reviewed-by: Benjamin Wright <benwright@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Benjamin Wright <benwright@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29883}
diff --git a/modules/audio_processing/audio_buffer.h b/modules/audio_processing/audio_buffer.h
index d27ccca..161c509 100644
--- a/modules/audio_processing/audio_buffer.h
+++ b/modules/audio_processing/audio_buffer.h
@@ -124,7 +124,8 @@
   void MergeFrequencyBands();
 
   // Copies the split bands data into the integer two-dimensional array.
-  void ExportSplitChannelData(size_t channel, int16_t* const* split_band_data);
+  void ExportSplitChannelData(size_t channel,
+                              int16_t* const* split_band_data) const;
 
   // Copies the data in the integer two-dimensional array into the split_bands
   // data.