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

webrtc/video/stats_counter.h

Index: webrtc/video/stats_counter.h
diff --git a/webrtc/video/stats_counter.h b/webrtc/video/stats_counter.h
index 08663f80155f1a6c582015e49e899e5550fa75eb..e84d73b758af20fedec697432ee6a4d945b45ffc 100644
--- a/webrtc/video/stats_counter.h
+++ b/webrtc/video/stats_counter.h
@@ -12,6 +12,7 @@
 #define WEBRTC_VIDEO_STATS_COUNTER_H_
 
 #include <memory>
+#include <string>
 
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/typedefs.h"
@@ -31,6 +32,8 @@ class StatsCounterObserver {
 };
 
 struct AggregatedStats {
+  std::string ToString() const;
+
   int64_t num_samples = 0;
   int min = -1;
   int max = -1;