Corrected the delay agnostic AEC behavior during periods of silent farend signal.

webrtc/modules/audio_processing/utility/delay_estimator.cc

Index: webrtc/modules/audio_processing/utility/delay_estimator.cc
diff --git a/webrtc/modules/audio_processing/utility/delay_estimator.cc b/webrtc/modules/audio_processing/utility/delay_estimator.cc
index 15a67472b176a0cd549c73db302bdc9b73eb3162..56bdde890c9348a0e2874950b15177a00ed72d0f 100644
--- a/webrtc/modules/audio_processing/utility/delay_estimator.cc
+++ b/webrtc/modules/audio_processing/utility/delay_estimator.cc
@@ -13,6 +13,7 @@
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
+#include <algorithm>
 
 // Number of right shifts for scaling is linearly depending on number of bits in
 // the far-end binary spectrum.
@@ -618,15 +619,29 @@ int WebRtc_ProcessBinarySpectrum(BinaryDelayEstimator* self,
       ((value_best_candidate < self->minimum_probability) ||
           (value_best_candidate < self->last_delay_probability)));
 
-  UpdateRobustValidationStatistics(self, candidate_delay, valley_depth,
-                                   value_best_candidate);
+  // Check for nonstationary farend signal.
+  const bool non_stationary_farend =
+      std::any_of(self->farend->far_bit_counts,
+                  self->farend->far_bit_counts + self->history_size,
+                  [](int a) { return a > 0; });
+
+  if (non_stationary_farend) {
+    // Only update the validation statistics when the farend is nonstationary
+    // as the underlying estimates are otherwise frozen.
+    UpdateRobustValidationStatistics(self, candidate_delay, valley_depth,
+                                     value_best_candidate);
+  }
+
   if (self->robust_validation_enabled) {
     int is_histogram_valid = HistogramBasedValidation(self, candidate_delay);
     valid_candidate = RobustValidation(self, candidate_delay, valid_candidate,
                                        is_histogram_valid);
 
   }
-  if (valid_candidate) {
+
+  // Only update the delay estimate when the farend is nonstationary and when
+  // a valid delay candidate is available.
+  if (non_stationary_farend && valid_candidate) {
     if (candidate_delay != self->last_delay) {
       self->last_delay_histogram =
           (self->histogram[candidate_delay] > kLastHistogramMax ?