Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/neteq/payload_splitter.cc

Index: webrtc/modules/audio_coding/neteq/payload_splitter.cc
diff --git a/webrtc/modules/audio_coding/neteq/payload_splitter.cc b/webrtc/modules/audio_coding/neteq/payload_splitter.cc
index 8530718134160341fff8519042f7b35f87d28fc5..ce13f5e2249943d6794d9af529091c7b2583d3a9 100644
--- a/webrtc/modules/audio_coding/neteq/payload_splitter.cc
+++ b/webrtc/modules/audio_coding/neteq/payload_splitter.cc
@@ -143,8 +143,9 @@ int PayloadSplitter::SplitFec(PacketList* packet_list,
 
     // Not an FEC packet.
     AudioDecoder* decoder = decoder_database->GetDecoder(payload_type);
-    // decoder should not return NULL.
-    assert(decoder != NULL);
+    // decoder should not return NULL, except for CNG as that's no longer a
+    // Decoder.

[kwiberg-webrtc, 2016/04/09 07:34:49]

Please avoid referring to how things used to be, since that'll eventually cease
to be enlightening.

  // This should either be CNG or a real decoder.
  RTC_DCHECK(decoder || decoder_database->IsComfortNoise(payload_type));

[ossu, 2016/04/11 08:31:10]

Acknowledged.

+    assert(decoder != NULL || decoder_database->IsComfortNoise(payload_type));
     if (!decoder ||
         !decoder->PacketHasFec(packet->payload, packet->payload_length)) {
       ++it;