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.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 };