Introduce NetEq::Config::ToString and use it in NetEq's constructor

R=minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/54559004

Cr-Commit-Position: refs/heads/master@{#9279}

webrtc/modules/audio_coding/neteq/interface/neteq.h

@@ -13,6 +13,7 @@
 
 #include <string.h>  // Provide access to size_t.
 
+#include <string>
 #include <vector>
 
 #include "webrtc/base/constructormagic.h"
@@ -81,6 +82,8 @@ class NetEq {
           background_noise_mode(kBgnOff),
           playout_mode(kPlayoutOn) {}
 
+    std::string ToString() const;
+
     int sample_rate_hz;  // Initial value. Will change with input data.
     bool enable_audio_classifier;
     int max_packets_in_buffer;