Pull out the PostFilter to its own NonlinearBeamformer API

webrtc/modules/audio_processing/beamformer/beamformer.h

 /*
  *  Copyright (c) 2015 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.
  */
 
 #ifndef WEBRTC_MODULES_AUDIO_PROCESSING_BEAMFORMER_BEAMFORMER_H_
 #define WEBRTC_MODULES_AUDIO_PROCESSING_BEAMFORMER_BEAMFORMER_H_
 
 #include "webrtc/common_audio/channel_buffer.h"
 #include "webrtc/modules/audio_processing/beamformer/array_util.h"
 
 namespace webrtc {
 
 template<typename T>
 class Beamformer {
  public:
   virtual ~Beamformer() {}
 
   // Process one time-domain chunk of audio. The audio is expected to be split
   // into frequency bands inside the ChannelBuffer. The number of frames and
-  // channels must correspond to the constructor parameters. The same
-  // ChannelBuffer can be passed in as |input| and |output|.
-  virtual void ProcessChunk(const ChannelBuffer<T>& input,
-                            ChannelBuffer<T>* output) = 0;
+  // channels must correspond to the constructor parameters.
+  virtual void ProcessChunk(ChannelBuffer<T>* data) = 0;
+
+  // Applies the postfilter mask to one chunk of audio. The audio is expected to
+  // be split into frequency bands inside the ChannelBuffer. The number of
+  // frames must correspond to the constructor parameters and the number of
+  // channels is expected to be 1, since that is the output number of channels
+  // of ProcessChunk().
+  virtual void PostFilter(ChannelBuffer<float>* data) {};
 
   // Sample rate corresponds to the lower band.
   // Needs to be called before the the Beamformer can be used.
   virtual void Initialize(int chunk_size_ms, int sample_rate_hz) = 0;
 
   // Aim the beamformer at a point in space.
   virtual void AimAt(const SphericalPointf& spherical_point) = 0;
 
   // Indicates whether a given point is inside of the beam.
   virtual bool IsInBeam(const SphericalPointf& spherical_point) { return true; }
 
   // Returns true if the current data contains the target signal.
   // Which signals are considered "targets" is implementation dependent.
   virtual bool is_target_present() = 0;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_PROCESSING_BEAMFORMER_BEAMFORMER_H_