message: test: use kCnCodecName instead of hardcoding "CN"
  Cr-Commit-Position: refs/heads/master@{#31482}
diff: @@ -825,8 +825,10 @@ TEST_P(PeerConnectionMediaTest, AnswerHasDifferentDirectionsForAudioVideo) {
  }
  void AddComfortNoiseCodecsToSend(cricket::FakeMediaEngine* media_engine) {
  - const cricket::AudioCodec kComfortNoiseCodec8k(102, "CN", 8000, 0, 1);
  - const cricket::AudioCodec kComfortNoiseCodec16k(103, "CN", 16000, 0, 1);
  + const cricket::AudioCodec kComfortNoiseCodec8k(102, cricket::kCnCodecName,
  + 8000, 0, 1);
  + const cricket::AudioCodec kComfortNoiseCodec16k(103, cricket::kCnCodecName,
  + 16000, 0, 1);
  auto codecs = media_engine->voice().send_codecs();
  codecs.push_back(kComfortNoiseCodec8k);
  @@ -837,7 +839,7 @@ void AddComfortNoiseCodecsToSend(cricket::FakeMediaEngine* media_engine) {
  bool HasAnyComfortNoiseCodecs(const cricket::SessionDescription* desc) {
  const auto* audio_desc = cricket::GetFirstAudioContentDescription(desc);
  for (const auto& codec : audio_desc->codecs()) {
  - if (codec.name == "CN") {
  + if (codec.name == cricket::kCnCodecName) {
  return true;
  }
  }
  