Update to current webrtc library

This is from the upstream library commit id
3326535126e435f1ba647885ce43a8f0f3d317eb, corresponding to Chromium
88.0.4290.1.

webrtc/modules/audio_processing/ns/signal_model.cc

@@ -0,0 +1,24 @@
+/*
+ *  Copyright (c) 2019 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "modules/audio_processing/ns/signal_model.h"
+
+namespace webrtc {
+
+SignalModel::SignalModel() {
+  constexpr float kSfFeatureThr = 0.5f;
+
+  lrt = kLtrFeatureThr;
+  spectral_flatness = kSfFeatureThr;
+  spectral_diff = kSfFeatureThr;
+  avg_log_lrt.fill(kLtrFeatureThr);
+}
+
+}  // namespace webrtc