Add empty residual echo detector.

webrtc/modules/audio_processing/include/audio_processing.h

Index: webrtc/modules/audio_processing/include/audio_processing.h
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index 0fdbfd2d75cf48caa50bb8b2f8a86f5e156ee353..76297172475ce0cd872ae1f8ae8a2063b8344fd5 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -258,6 +258,9 @@ class AudioProcessing {
       // the allowed range is [-100, 0].
       float initial_peak_level_dbfs = -6.0206f;
     } level_controller;
+    struct ResidualEchoDetector {
+      bool enabled = false;
+    } residual_echo_detector;
   };
 
   // TODO(mgraczyk): Remove once all methods that use ChannelLayout are gone.