Supporting Opus DTX in Voice Engine.

Opus DTX is an Opus specific feature. It does not require WebRTC VAD/DTX, therefore is not set by VoECodec::SetVADStatus(), but rather a dedicated API.

BUG=1014
R=henrika@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43709004

Cr-Commit-Position: refs/heads/master@{#8716}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8716 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/voice_engine/voe_codec_impl.h

@@ -56,6 +56,8 @@ public:
 
     virtual int SetOpusMaxPlaybackRate(int channel, int frequency_hz);
 
+    virtual int SetOpusDtx(int channel, bool enable_dtx);
+
 protected:
     VoECodecImpl(voe::SharedData* shared);
     virtual ~VoECodecImpl();