Added the ability to adjust the AEC3 performance for large rooms

This CL exposes the parameter for adjusting the AEC3 performance
for large rooms.

Bug: webrtc:7519
Review-Url: https://codereview.webrtc.org/2967603002
Cr-Commit-Position: refs/heads/master@{#18862}
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index 05be5fe..6dc6e3f 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -268,6 +268,7 @@
     // does not yet have the desired behavior.
     struct EchoCanceller3 {
       bool enabled = false;
+      float echo_decay = 0.f;
     } echo_canceller3;
 
     // Enables the next generation AGC functionality. This feature replaces the