Added the ability to set the default echo path delay in AEC3.

This CL adds the ability to set a default echo path delay to use
in AEC3 when there is prior knowledge about the delay in the echo
path.


Bug: webrtc:8358
Change-Id: Ie368f9a6dec9f412e09bf0e095f89d84305045f9
Reviewed-on: https://webrtc-review.googlesource.com/7604
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20204}

modules/audio_processing/include/audio_processing.h

@@ -272,6 +272,10 @@ class AudioProcessing : public rtc::RefCountInterface {
     // does not yet have the desired behavior.
     struct EchoCanceller3 {
       struct Param {
+        struct Delay {
+          size_t default_delay = 5;
+        } delay;
+
         struct Erle {
           float min = 1.f;
           float max_l = 8.f;