Added metric for the delay in AEC3.

Bug: webrtc:8569
Change-Id: I659049a411654bd3a252ab29008fac467f903efd
Reviewed-on: https://webrtc-review.googlesource.com/25600
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20892}

modules/audio_processing/include/audio_processing_statistics.h

@@ -44,6 +44,11 @@ struct AudioProcessingStats {
   rtc::Optional<double> residual_echo_likelihood;
   // Maximum residual echo likelihood from the last time period.
   rtc::Optional<double> residual_echo_likelihood_recent_max;
+
+  // The instantaneous delay estimate produced in the AEC. The unit is in
+  // milliseconds and the value is the instantaneous value at the time of the
+  // call to |GetStatistics()|.
+  int delay_ms;
 };
 
 }  // namespace webrtc