Corrected the number of channels used when AEC3 is run on stereo input.

webrtc/modules/audio_processing/audio_processing_impl.cc

Index: webrtc/modules/audio_processing/audio_processing_impl.cc
diff --git a/webrtc/modules/audio_processing/audio_processing_impl.cc b/webrtc/modules/audio_processing/audio_processing_impl.cc
index 816210f34f8738eb7a042adee69f87f5ed1a99d8..c408505f66908a5e0b150a189ec9d9007462dce1 100644
--- a/webrtc/modules/audio_processing/audio_processing_impl.cc
+++ b/webrtc/modules/audio_processing/audio_processing_impl.cc
@@ -725,7 +725,10 @@ size_t AudioProcessingImpl::num_input_channels() const {
 
 size_t AudioProcessingImpl::num_proc_channels() const {
   // Used as callback from submodules, hence locking is not allowed.
-  return capture_nonlocked_.beamformer_enabled ? 1 : num_output_channels();
+  return (capture_nonlocked_.beamformer_enabled ||
+          capture_nonlocked_.echo_canceller3_enabled)
+             ? 1
+             : num_output_channels();
 }
 
 size_t AudioProcessingImpl::num_output_channels() const {