Regression test for issue where Opus DTX status was being forgotten.

BUG=webrtc:6020

Review-Url: https://codereview.webrtc.org/2177263002
Cr-Commit-Position: refs/heads/master@{#13539}

webrtc/modules/audio_coding/acm2/audio_coding_module.cc

@@ -48,6 +48,8 @@ class AudioCodingModuleImpl final : public AudioCodingModule {
   void ModifyEncoder(
       FunctionView<void(std::unique_ptr<AudioEncoder>*)> modifier) override;
 
+  void QueryEncoder(FunctionView<void(const AudioEncoder*)> query) override;
+
   // Get current send codec.
   rtc::Optional<CodecInst> SendCodec() const override;
 
@@ -596,6 +598,12 @@ void AudioCodingModuleImpl::ModifyEncoder(
   modifier(&encoder_stack_);
 }
 
+void AudioCodingModuleImpl::QueryEncoder(
+    FunctionView<void(const AudioEncoder*)> query) {
+  rtc::CritScope lock(&acm_crit_sect_);
+  query(encoder_stack_.get());
+}
+
 // Get current send codec.
 rtc::Optional<CodecInst> AudioCodingModuleImpl::SendCodec() const {
   rtc::CritScope lock(&acm_crit_sect_);

webrtc/modules/audio_coding/codecs/audio_encoder.cc

@@ -50,6 +50,10 @@ bool AudioEncoder::SetDtx(bool enable) {
   return !enable;
 }
 
+bool AudioEncoder::GetDtx() const {
+  return false;
+}
+
 bool AudioEncoder::SetApplication(Application application) {
   return false;
 }

webrtc/modules/audio_coding/codecs/audio_encoder.h

@@ -127,6 +127,10 @@ class AudioEncoder {
   // supported).
   virtual bool SetDtx(bool enable);
 
+  // Returns the status of codec-internal DTX. The default implementation always
+  // returns false.
+  virtual bool GetDtx() const;
+
   // Sets the application mode. Returns true if the codec was able to comply.
   // The default implementation just returns false.
   enum class Application { kSpeech, kAudio };

webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc

@@ -152,6 +152,10 @@ bool AudioEncoderOpus::SetDtx(bool enable) {
   return RecreateEncoderInstance(conf);
 }
 
+bool AudioEncoderOpus::GetDtx() const {
+  return config_.dtx_enabled;
+}
+
 bool AudioEncoderOpus::SetApplication(Application application) {
   auto conf = config_;
   switch (application) {

webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.h

@@ -75,6 +75,7 @@ class AudioEncoderOpus final : public AudioEncoder {
   // being inactive. During that, it still sends 2 packets (one for content, one
   // for signaling) about every 400 ms.
   bool SetDtx(bool enable) override;
+  bool GetDtx() const override;
 
   bool SetApplication(Application application) override;
   void SetMaxPlaybackRate(int frequency_hz) override;
@@ -84,7 +85,6 @@ class AudioEncoderOpus final : public AudioEncoder {
   // Getters for testing.
   double packet_loss_rate() const { return packet_loss_rate_; }
   ApplicationMode application() const { return config_.application; }
-  bool dtx_enabled() const { return config_.dtx_enabled; }
 
  protected:
   EncodedInfo EncodeImpl(uint32_t rtp_timestamp,

webrtc/modules/audio_coding/include/audio_coding_module.h

@@ -250,6 +250,10 @@ class AudioCodingModule {
   virtual void ModifyEncoder(
       FunctionView<void(std::unique_ptr<AudioEncoder>*)> modifier) = 0;
 
+  // |modifier| is called exactly once with one argument: a const pointer to the
+  // current encoder (which is null if there is no current encoder).
+  virtual void QueryEncoder(FunctionView<void(AudioEncoder const*)> query) = 0;
+
   // Utility method for simply replacing the existing encoder with a new one.
   void SetEncoder(std::unique_ptr<AudioEncoder> new_encoder) {
     ModifyEncoder([&](std::unique_ptr<AudioEncoder>* encoder) {