message: configure target bitrate in opus dtx tests
  This avoids a difference in behaviour between mobile and
  desktop platforms since the bitrate is now too low for
  CELT mode.
  Cr-Commit-Position: refs/heads/master@{#31757}
diff: @@ -267,16 +267,15 @@ void TestOpusDtx::Perform() {
  acm_send_->ModifyEncoder([](std::unique_ptr<AudioEncoder>* encoder_ptr) {
  (*encoder_ptr)->SetDtx(true);
  + // The default bitrate will not generate frames recognized as CN on desktop
  + // since the frames will be encoded as CELT. Set a low target bitrate to get
  + // consistent behaviour across platforms.
  + (*encoder_ptr)->OnReceivedTargetAudioBitrate(24000);
  });
  expects[static_cast<int>(AudioFrameType::kEmptyFrame)] = 1;
  expects[static_cast<int>(AudioFrameType::kAudioFrameSpeech)] = 1;
  - // Android and iOS behave different with respect to the number of CN frames.
  -#if defined(WEBRTC_IOS) || defined(WEBRTC_ANDROID)
  expects[static_cast<int>(AudioFrameType::kAudioFrameCN)] = 1;
  -#else
  - expects[static_cast<int>(AudioFrameType::kAudioFrameCN)] = 0;
  -#endif
  Run(webrtc::test::ResourcePath("audio_coding/teststereo32kHz", "pcm"), 32000,
  2, out_filename, true, expects);
  }
  