Add algorithm for Residual Echo Detector.

webrtc/modules/BUILD.gn

Index: webrtc/modules/BUILD.gn
diff --git a/webrtc/modules/BUILD.gn b/webrtc/modules/BUILD.gn
index ee18569ff8c5cc395150ac19b2fc62ffb1b0c59c..df24e5b8be659837c6fcee84960bba274fb5aebe 100644
--- a/webrtc/modules/BUILD.gn
+++ b/webrtc/modules/BUILD.gn
@@ -586,11 +586,15 @@ if (rtc_include_tests) {
         "audio_processing/audio_processing_unittest.cc",
         "audio_processing/echo_cancellation_bit_exact_unittest.cc",
         "audio_processing/echo_control_mobile_unittest.cc",
+        "audio_processing/echo_detector/circular_buffer_unittest.cc",
+        "audio_processing/echo_detector/mean_variance_estimator_unittest.cc",
+        "audio_processing/echo_detector/normalized_covariance_estimator_unittest.cc",
         "audio_processing/gain_control_unittest.cc",
         "audio_processing/high_pass_filter_unittest.cc",
         "audio_processing/level_controller/level_controller_unittest.cc",
         "audio_processing/level_estimator_unittest.cc",
         "audio_processing/noise_suppression_unittest.cc",
+        "audio_processing/residual_echo_detector_unittest.cc",
         "audio_processing/test/bitexactness_tools.cc",
         "audio_processing/test/bitexactness_tools.h",
         "audio_processing/test/debug_dump_replayer.cc",