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..9d799a34971c944e990f678a28982bdf2e056db8 100644
--- a/webrtc/modules/audio_processing/utility/delay_estimator.cc
+++ b/webrtc/modules/audio_processing/utility/delay_estimator.cc
@@ -618,15 +618,32 @@ 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.
+  bool non_stationary_farend = false;

[hlundin-webrtc, 2016/05/11 12:39:48]

You are traversing the array from end to beginning. If this is not important, I
suggest a one-liner instead:

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; });

I don't think it works to use any_of to backwards-traverse a raw array. One
option is to wrap the array (just locally) in an ArrayView and get iterators out
of it, but ArrayView currently has no reverse-iterators implemented.

[peah-webrtc, 2016/05/11 12:53:33]

Wow! That was great!! And works splendidly. Thanks!
Done.

+  for (i = self->history_size - 1; i >= 0; --i) {
+    if (self->farend->far_bit_counts[i] > 0) {
+      non_stationary_farend = true;
+      break;
+    }
+  }
+
+  if (non_stationary_farend) {
+    // Only update the validation statistics when the farend is nonstationary
+    // as the underlying estimates are otherwise freezed.

[hlundin-webrtc, 2016/05/11 12:39:48]

frozen

[peah-webrtc, 2016/05/11 12:53:33]

Done.

+    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 ?