Remove additional channel constraints when Beamforming is enabled in AudioProcessing

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 c1c4061f4884c727866fd185ec394974a4ed7bb8..77bda79a0c49411b43f3151b78addd48b92f8eaf 100644
--- a/webrtc/modules/audio_processing/audio_buffer.cc
+++ b/webrtc/modules/audio_processing/audio_buffer.cc
@@ -150,7 +150,7 @@ void AudioBuffer::CopyFrom(const float* const* data,
 void AudioBuffer::CopyTo(const StreamConfig& stream_config,
                          float* const* data) {
   assert(stream_config.num_frames() == output_num_frames_);
-  assert(stream_config.num_channels() == num_channels_);
+  assert(stream_config.num_channels() == num_channels_ || num_channels_ == 1);
 
   // Convert to the float range.
   float* const* data_ptr = data;
@@ -173,6 +173,11 @@ void AudioBuffer::CopyTo(const StreamConfig& stream_config,
                                       output_num_frames_);
     }
   }
+
+  // Upmix.
+  for (int i = num_channels_; i < stream_config.num_channels(); ++i) {
+    memcpy(data[i], data[0], output_num_frames_ * sizeof(**data));
+  }
 }
 
 void AudioBuffer::InitForNewData() {