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..be3d7cd21c033eca6dae3c3897f69fdc6d869d03 100644
--- a/webrtc/modules/audio_processing/beamformer/beamformer.h
+++ b/webrtc/modules/audio_processing/beamformer/beamformer.h
@@ -23,10 +23,15 @@ class 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.