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}
diff --git a/webrtc/modules/audio_coding/codecs/audio_encoder.cc b/webrtc/modules/audio_coding/codecs/audio_encoder.cc
index 6b7f5f8..c433dcd 100644
--- a/webrtc/modules/audio_coding/codecs/audio_encoder.cc
+++ b/webrtc/modules/audio_coding/codecs/audio_encoder.cc
@@ -50,6 +50,10 @@
   return !enable;
 }
 
+bool AudioEncoder::GetDtx() const {
+  return false;
+}
+
 bool AudioEncoder::SetApplication(Application application) {
   return false;
 }
diff --git a/webrtc/modules/audio_coding/codecs/audio_encoder.h b/webrtc/modules/audio_coding/codecs/audio_encoder.h
index ecc28d9..f09525f 100644
--- a/webrtc/modules/audio_coding/codecs/audio_encoder.h
+++ b/webrtc/modules/audio_coding/codecs/audio_encoder.h
@@ -127,6 +127,10 @@
   // 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 };
diff --git a/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc b/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
index edafbbd..d03f2d3 100644
--- a/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
+++ b/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
@@ -152,6 +152,10 @@
   return RecreateEncoderInstance(conf);
 }
 
+bool AudioEncoderOpus::GetDtx() const {
+  return config_.dtx_enabled;
+}
+
 bool AudioEncoderOpus::SetApplication(Application application) {
   auto conf = config_;
   switch (application) {
diff --git a/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.h b/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.h
index 367c9b9..48fb494 100644
--- a/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.h
+++ b/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.h
@@ -75,6 +75,7 @@
   // 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 @@
   // 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,