Add empty residual echo detector.

webrtc/modules/audio_processing/audio_processing.gypi

 # Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 {
   'variables': {
@@ skipping 93 matching lines @@
         'level_controller/saturating_gain_estimator.h',
         'level_controller/signal_classifier.cc',
         'level_controller/signal_classifier.h',
         'level_estimator_impl.cc',
         'level_estimator_impl.h',
         'logging/apm_data_dumper.cc',
         'logging/apm_data_dumper.h',
         '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',
         'splitting_filter.h',
         'three_band_filter_bank.cc',
         'three_band_filter_bank.h',
         'transient/common.h',
         'transient/daubechies_8_wavelet_coeffs.h',
         'transient/dyadic_decimator.h',
         'transient/moving_moments.cc',
@@ skipping 191 matching lines @@
             'defines': ['WEBRTC_APM_DEBUG_DUMP=1',],
           }],
           ['apm_debug_dump==0', {
             'defines': ['WEBRTC_APM_DEBUG_DUMP=0',],
           }],
         ],
       }],
     }],
   ],
 }