Adding googAudioNetworkAdaptorConfig to MediaConstraintsInterface.

This is to allow application to pass an audio network adaptor config string to WebRTC.

BUG=webrtc:6303

Review-Url: https://codereview.webrtc.org/2437803004
Cr-Commit-Position: refs/heads/master@{#15532}
diff --git a/webrtc/api/mediaconstraintsinterface.h b/webrtc/api/mediaconstraintsinterface.h
index fbb1955..a5d16cb 100644
--- a/webrtc/api/mediaconstraintsinterface.h
+++ b/webrtc/api/mediaconstraintsinterface.h
@@ -80,6 +80,8 @@
   static const char kHighpassFilter[];  // googHighpassFilter
   static const char kTypingNoiseDetection[];  // googTypingNoiseDetection
   static const char kAudioMirroring[];  // googAudioMirroring
+  static const char
+      kAudioNetworkAdaptorConfig[];  // goodAudioNetworkAdaptorConfig
 
   // Google-specific constraint keys for a local video source
   static const char kNoiseReduction[];  // googNoiseReduction