Pull out the PostFilter to its own NonlinearBeamformer API

webrtc/modules/audio_processing/audio_buffer.cc

Index: webrtc/modules/audio_processing/audio_buffer.cc
diff --git a/webrtc/modules/audio_processing/audio_buffer.cc b/webrtc/modules/audio_processing/audio_buffer.cc
index f9b9f631d2b813523ed4a9935ecbc3c0af2ab3bd..13ece67f254f9fcdc5115a58db28e9de411954f5 100644
--- a/webrtc/modules/audio_processing/audio_buffer.cc
+++ b/webrtc/modules/audio_processing/audio_buffer.cc
@@ -184,6 +184,10 @@ void AudioBuffer::InitForNewData() {
   reference_copied_ = false;
   activity_ = AudioFrame::kVadUnknown;
   num_channels_ = num_proc_channels_;
+  data_->set_num_channels(num_proc_channels_);
+  if (split_data_.get()) {
+    split_data_->set_num_channels(num_proc_channels_);
+  }
 }
 
 const int16_t* const* AudioBuffer::channels_const() const {
@@ -345,6 +349,10 @@ size_t AudioBuffer::num_channels() const {
 
 void AudioBuffer::set_num_channels(size_t num_channels) {
   num_channels_ = num_channels;
+  data_->set_num_channels(num_channels);
+  if (split_data_.get()) {
+    split_data_->set_num_channels(num_channels);
+  }
 }
 
 size_t AudioBuffer::num_frames() const {