Pull out the PostFilter to its own NonlinearBeamformer API

webrtc/common_audio/lapped_transform.cc

Index: webrtc/common_audio/lapped_transform.cc
diff --git a/webrtc/common_audio/lapped_transform.cc b/webrtc/common_audio/lapped_transform.cc
index 5ab1db1b2586142631768f5625014b894f5749fa..006bda0cd1af68a317cd3a0876964cb11e383d7a 100644
--- a/webrtc/common_audio/lapped_transform.cc
+++ b/webrtc/common_audio/lapped_transform.cc
@@ -83,7 +83,7 @@ LappedTransform::LappedTransform(size_t num_in_channels,
       cplx_post_(num_out_channels,
                  cplx_length_,
                  RealFourier::kFftBufferAlignment) {
-  RTC_CHECK(num_in_channels_ > 0 && num_out_channels_ > 0);
+  RTC_CHECK(num_in_channels_ > 0);
   RTC_CHECK_GT(block_length_, 0u);
   RTC_CHECK_GT(chunk_length_, 0u);
   RTC_CHECK(block_processor_);