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/fft_buffer.cc

@@ -12,7 +12,7 @@
 
 namespace webrtc {
 
-FftBuffer::FftBuffer(size_t size) : buffer(size) {
+FftBuffer::FftBuffer(size_t size) : size(static_cast<int>(size)), buffer(size) {
   for (auto& b : buffer) {
     b.Clear();
   }