Pull out the PostFilter to its own NonlinearBeamformer API

webrtc/modules/audio_processing/include/audio_processing.h

Index: webrtc/modules/audio_processing/include/audio_processing.h
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index d25c2525b30d485cb5d0d6dde04c04449e5157dc..06bfc9ba7f9d8531044105ea4ddd3790cbf90a31 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -31,8 +31,7 @@ struct AecCore;
 
 class AudioFrame;
 
-template<typename T>
-class Beamformer;
+class NonlinearBeamformer;
 
 class StreamConfig;
 class ProcessingConfig;
@@ -275,7 +274,7 @@ class AudioProcessing {
   static AudioProcessing* Create(const Config& config);
   // Only for testing.
   static AudioProcessing* Create(const Config& config,
-                                 Beamformer<float>* beamformer);
+                                 NonlinearBeamformer* beamformer);
   virtual ~AudioProcessing() {}
 
   // Initializes internal states, while retaining all user settings. This