Attach TransportFeedbackPacketLossTracker to ANA (PLR only)

This CL is one in a series. To finish the work, the following CLs will be added:
1. CL for connecting RPLR as well
2. CL for RPLR-based FecController
3. CL for allowing experiment-driven configuration of the above (through both field-trials and protobuf)

BUG=webrtc:7058

Review-Url: https://codereview.webrtc.org/2638083002
Cr-Commit-Position: refs/heads/master@{#17365}

webrtc/voice_engine/channel_proxy.h

@@ -116,6 +116,7 @@ class ChannelProxy {
   virtual bool SetOpusMaxPlaybackRate(int frequency_hz);
   virtual bool SetSendCodec(const CodecInst& codec_inst);
   virtual bool SetSendCNPayloadType(int type, PayloadFrequencies frequency);
+  virtual void OnTwccBasedUplinkPacketLossRate(float packet_loss_rate);
 
  private:
   Channel* channel() const;