Pull out the PostFilter to its own NonlinearBeamformer API

webrtc/modules/audio_processing/beamformer/beamformer.h

Index: webrtc/modules/audio_processing/beamformer/beamformer.h
diff --git a/webrtc/modules/audio_processing/beamformer/beamformer.h b/webrtc/modules/audio_processing/beamformer/beamformer.h
index 6a9ff45d129e14d84676db9b068b414f26850c54..9096449d6789b5fe3ae99a203848ceaa44c4f9c8 100644
--- a/webrtc/modules/audio_processing/beamformer/beamformer.h
+++ b/webrtc/modules/audio_processing/beamformer/beamformer.h
@@ -28,6 +28,13 @@ class Beamformer {
   virtual void ProcessChunk(const ChannelBuffer<T>& input,
                             ChannelBuffer<T>* output) = 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;