Move the stereo-disables-CNG logic from CodecManager to Rent-A-Codec

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1473563004

Cr-Original-Commit-Position: refs/heads/master@{#10784}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 989b4abcf32d4a1605e7af160a1a07916c6b324e
diff --git a/modules/audio_coding/main/acm2/codec_manager.cc b/modules/audio_coding/main/acm2/codec_manager.cc
index 8022935..7db6919 100644
--- a/modules/audio_coding/main/acm2/codec_manager.cc
+++ b/modules/audio_coding/main/acm2/codec_manager.cc
@@ -149,15 +149,6 @@
       break;
   }
 
-  // Set Stereo, and make sure VAD and DTX is turned off.
-  if (send_codec.channels != 1) {
-    if (codec_stack_params_.use_cng) {
-      WEBRTC_TRACE(webrtc::kTraceWarning, webrtc::kTraceAudioCoding, dummy_id,
-                   "VAD/DTX is turned off, not supported when sending stereo.");
-    }
-    codec_stack_params_.use_cng = false;
-  }
-
   // Check if the codec is already registered as send codec.
   bool new_codec = true;
   if (CurrentEncoder()) {
@@ -231,8 +222,6 @@
   static const char kName[] = "external";
   memcpy(send_codec_inst_.plname, kName, sizeof(kName));
 
-  if (send_codec_inst_.channels != 1)
-    codec_stack_params_.use_cng = false;
   if (codec_stack_params_.use_codec_fec) {
     // Switch FEC on. On failure, remember that FEC is off.
     if (!external_speech_encoder->SetFec(true))
diff --git a/modules/audio_coding/main/acm2/rent_a_codec.cc b/modules/audio_coding/main/acm2/rent_a_codec.cc
index 3281814..b31092c 100644
--- a/modules/audio_coding/main/acm2/rent_a_codec.cc
+++ b/modules/audio_coding/main/acm2/rent_a_codec.cc
@@ -259,7 +259,8 @@
                          : rtc::Optional<int>(it->second);
   };
   auto cng_pt = pt(param->cng_payload_types);
-  param->use_cng = param->use_cng && cng_pt;
+  param->use_cng =
+      param->use_cng && cng_pt && speech_encoder->NumChannels() == 1;
   auto red_pt = pt(param->red_payload_types);
   param->use_red = param->use_red && red_pt;