Added the ability to more easily adjust the filter length in AEC3

Bug: webrtc:8609
Change-Id: If060b332993c2c98d7a12608ab31f4da858b8016
Reviewed-on: https://webrtc-review.googlesource.com/28620
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21216}

modules/audio_processing/include/audio_processing.h

@@ -1158,6 +1158,10 @@ struct EchoCanceller3Config {
     size_t min_echo_path_delay_blocks = 5;
   } delay;
 
+  struct Filter {
+    size_t length_blocks = 12;
+  } filter;
+
   struct Erle {
     float min = 1.f;
     float max_l = 8.f;