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;
 }