Make VectorBuffer in AEC3 multi-channel

All dependent modules are hardcoded to do their regular mono processing on the first channel.

This _almost_ makes RenderBuffer multi-channel: FftData is still only mono.

Bug: webrtc:10913
Change-Id: Id5cc34dbabfe59e1cc72a9675dc7979794e870ed
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151139
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Per Ã…hgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29074}
diff --git a/modules/audio_processing/aec3/render_buffer.h b/modules/audio_processing/aec3/render_buffer.h
index 8759760..7c70e1b 100644
--- a/modules/audio_processing/aec3/render_buffer.h
+++ b/modules/audio_processing/aec3/render_buffer.h
@@ -44,10 +44,11 @@
   }
 
   // Get the spectrum from one of the FFTs in the buffer.
-  rtc::ArrayView<const float> Spectrum(int buffer_offset_ffts) const {
+  rtc::ArrayView<const float> Spectrum(int buffer_offset_ffts,
+                                       size_t channel) const {
     int position = spectrum_buffer_->OffsetIndex(spectrum_buffer_->read,
                                                  buffer_offset_ffts);
-    return spectrum_buffer_->buffer[position];
+    return spectrum_buffer_->buffer[position][channel];
   }
 
   // Returns the circular fft buffer.