Corrections of the render buffering scheme in AEC3 to ensure causality

This CL modifies the refactored render buffering scheme in AEC3
so that:
-A non-causal state can never occur which means that situations with
 nonrecoverable echo should not occur.
-For a stable audio pipeline with a predefined API call jitter,
 render overruns and underruns can never occur.

Bug: webrtc:8629,chromium:793305
Change-Id: I06ba1c368f92db95274090b08475dd02dbb85145
Reviewed-on: https://webrtc-review.googlesource.com/29861
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21215}

modules/audio_processing/aec3/render_buffer.h

@@ -64,7 +64,7 @@ class RenderBuffer {
 
  private:
   const MatrixBuffer* const block_buffer_;
-  VectorBuffer* spectrum_buffer_;
+  const VectorBuffer* const spectrum_buffer_;
   const FftBuffer* const fft_buffer_;
   const size_t spectral_sums_length_;
   std::array<float, kFftLengthBy2Plus1> spectral_sums_;