Add a placeholder stat for logging the estimated residual echo likelihood.

webrtc/media/base/mediachannel.h

Index: webrtc/media/base/mediachannel.h
diff --git a/webrtc/media/base/mediachannel.h b/webrtc/media/base/mediachannel.h
index b71a309233546f941150431bf5df4aca8f46b095..b797e7a387540e5bbfa17b0aa4c34281b3c15e52 100644
--- a/webrtc/media/base/mediachannel.h
+++ b/webrtc/media/base/mediachannel.h
@@ -587,6 +587,7 @@ struct VoiceSenderInfo : public MediaSenderInfo {
         echo_delay_std_ms(0),
         echo_return_loss(0),
         echo_return_loss_enhancement(0),
+        residual_echo_likelihood(0.0f),
         typing_noise_detected(false) {
   }
 
@@ -598,6 +599,7 @@ struct VoiceSenderInfo : public MediaSenderInfo {
   int echo_delay_std_ms;
   int echo_return_loss;
   int echo_return_loss_enhancement;
+  float residual_echo_likelihood;
   bool typing_noise_detected;
 };