Add ToString method to AggregatedStats and log stats at the end of a call.

BUG=webrtc:5283

Review-Url: https://codereview.webrtc.org/2494423002
Cr-Commit-Position: refs/heads/master@{#15088}

webrtc/video/send_delay_stats.cc

@@ -45,6 +45,7 @@ void SendDelayStats::UpdateHistograms() {
     AggregatedStats stats = it.second->GetStats();
     if (stats.num_samples >= kMinRequiredPeriodicSamples) {
       RTC_HISTOGRAM_COUNTS_10000("WebRTC.Video.SendDelayInMs", stats.average);
+      LOG(LS_INFO) << "WebRTC.Video.SendDelayInMs, " << stats.ToString();
     }
   }
 }