Remove additional channel constraints when Beamforming is enabled in AudioProcessing

The general constraints on number of channels for AudioProcessing is:
num_in_channels == num_out_channels || num_out_channels == 1

When Beamforming is enabled and additional constraint was added forcing:
num_out_channels == 1

This artificial constraint was removed by adding upmixing support in CopyTo, since it was already supported for the AudioFrame interface using InterleaveTo.

Review URL: https://codereview.webrtc.org/1571013002

Cr-Commit-Position: refs/heads/master@{#11215}
diff --git a/webrtc/modules/audio_processing/gain_control_impl.cc b/webrtc/modules/audio_processing/gain_control_impl.cc
index b9b3564..7b284e8 100644
--- a/webrtc/modules/audio_processing/gain_control_impl.cc
+++ b/webrtc/modules/audio_processing/gain_control_impl.cc
@@ -435,7 +435,7 @@
 
 int GainControlImpl::num_handles_required() const {
   // Not locked as it only relies on APM public API which is threadsafe.
-  return apm_->num_output_channels();
+  return apm_->num_proc_channels();
 }
 
 int GainControlImpl::GetHandleError(void* handle) const {