Add empty residual echo detector.

webrtc/modules/audio_processing/BUILD.gn

Index: webrtc/modules/audio_processing/BUILD.gn
diff --git a/webrtc/modules/audio_processing/BUILD.gn b/webrtc/modules/audio_processing/BUILD.gn
index 25c47ec9641c8e957e148d8c341184b0d892852a..7647161406013ff7ee59249d4e99dd21b8152fd5 100644
--- a/webrtc/modules/audio_processing/BUILD.gn
+++ b/webrtc/modules/audio_processing/BUILD.gn
@@ -61,6 +61,8 @@ rtc_static_library("audio_processing") {
     "echo_cancellation_impl.h",
     "echo_control_mobile_impl.cc",
     "echo_control_mobile_impl.h",
+    "echo_detector/echo_detector.cc",
+    "echo_detector/echo_detector.h",
     "gain_control_for_experimental_agc.cc",
     "gain_control_for_experimental_agc.h",
     "gain_control_impl.cc",
@@ -99,6 +101,8 @@ rtc_static_library("audio_processing") {
     "noise_suppression_impl.cc",
     "noise_suppression_impl.h",
     "render_queue_item_verifier.h",
+    "residual_echo_detector.cc",
+    "residual_echo_detector.h",
     "rms_level.cc",
     "rms_level.h",
     "splitting_filter.cc",