Posting Opus's set-force-channels functionality to WebRTC.

webrtc/modules/audio_coding/codecs/opus/opus_interface.c

Index: webrtc/modules/audio_coding/codecs/opus/opus_interface.c
diff --git a/webrtc/modules/audio_coding/codecs/opus/opus_interface.c b/webrtc/modules/audio_coding/codecs/opus/opus_interface.c
index d79a9ad290842db28ab62a14867bfe0a21684c8b..aff089f0923eea8cd8d6f70ba66cf9f987a3566f 100644
--- a/webrtc/modules/audio_coding/codecs/opus/opus_interface.c
+++ b/webrtc/modules/audio_coding/codecs/opus/opus_interface.c
@@ -208,6 +208,20 @@ int16_t WebRtcOpus_SetComplexity(OpusEncInst* inst, int32_t complexity) {
   }
 }
 
+int16_t WebRtcOpus_SetForceChannels(OpusEncInst* inst, int32_t num_channels) {
+  if (!inst)
+    return -1;
+  if (num_channels == 0) {
+    return opus_encoder_ctl(inst->encoder,
+                            OPUS_SET_FORCE_CHANNELS(OPUS_AUTO));
+  } else if (num_channels == 1 || num_channels == 2) {
+    return opus_encoder_ctl(inst->encoder,
+                            OPUS_SET_FORCE_CHANNELS(num_channels));
+  } else {
+    return -1;
+  }
+}
+
 int16_t WebRtcOpus_DecoderCreate(OpusDecInst** inst, size_t channels) {
   int error;
   OpusDecInst* state;