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/voice_engine/voe_codec_impl.h b/webrtc/voice_engine/voe_codec_impl.h
index 5519232..d24bbac 100644
--- a/webrtc/voice_engine/voe_codec_impl.h
+++ b/webrtc/voice_engine/voe_codec_impl.h
@@ -58,6 +58,8 @@
 
   int SetOpusDtx(int channel, bool enable_dtx) override;
 
+  int GetOpusDtxStatus(int channel, bool* enabled) override;
+
  protected:
   VoECodecImpl(voe::SharedData* shared);
   ~VoECodecImpl() override;