Configure VoE NACK through AudioSendStream::Config.

webrtc/config.h

Index: webrtc/config.h
diff --git a/webrtc/config.h b/webrtc/config.h
index 083e778a4153b38371edc5f02c778bc16a6c158c..887f021498e1347b2591bdbbbb6cb8de7585bcb0 100644
--- a/webrtc/config.h
+++ b/webrtc/config.h
@@ -25,6 +25,7 @@ namespace webrtc {
 // Settings for NACK, see RFC 4585 for details.
 struct NackConfig {
   NackConfig() : rtp_history_ms(0) {}
+  std::string ToString() const;
   // Send side: the time RTP packets are stored for retransmissions.
   // Receive side: the time the receiver is prepared to wait for
   // retransmissions.