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.cc b/modules/audio_processing/audio_buffer.cc
index 81ded91..9f79b54 100644
--- a/modules/audio_processing/audio_buffer.cc
+++ b/modules/audio_processing/audio_buffer.cc
@@ -369,10 +369,11 @@
   splitting_filter_->Synthesis(split_data_.get(), data_.get());
 }
 
-void AudioBuffer::ExportSplitChannelData(size_t channel,
-                                         int16_t* const* split_band_data) {
+void AudioBuffer::ExportSplitChannelData(
+    size_t channel,
+    int16_t* const* split_band_data) const {
   for (size_t k = 0; k < num_bands(); ++k) {
-    const float* band_data = split_bands(channel)[k];
+    const float* band_data = split_bands_const(channel)[k];
 
     RTC_DCHECK(split_band_data[k]);
     RTC_DCHECK(band_data);