Pull out the PostFilter to its own NonlinearBeamformer API

webrtc/modules/audio_processing/BUILD.gn

 # Copyright (c) 2014 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.
 
 import("//build/config/arm.gni")
 import("//third_party/protobuf/proto_library.gni")
@@ skipping 37 matching lines @@
     "agc/loudness_histogram.cc",
     "agc/loudness_histogram.h",
     "agc/utility.cc",
     "agc/utility.h",
     "audio_buffer.cc",
     "audio_buffer.h",
     "audio_processing_impl.cc",
     "audio_processing_impl.h",
     "beamformer/array_util.cc",
     "beamformer/array_util.h",
-    "beamformer/beamformer.h",
     "beamformer/complex_matrix.h",
     "beamformer/covariance_matrix_generator.cc",
     "beamformer/covariance_matrix_generator.h",
     "beamformer/matrix.h",
     "beamformer/nonlinear_beamformer.cc",
     "beamformer/nonlinear_beamformer.h",
     "common.h",
     "echo_cancellation_impl.cc",
     "echo_cancellation_impl.h",
     "echo_control_mobile_impl.cc",
@@ skipping 282 matching lines @@
         "test/protobuf_utils.cc",
         "test/protobuf_utils.h",
       ]
 
       deps = [
         ":audioproc_unittest_proto",
       ]
     }
   }
 }