Make RTCStatsReport::ToString() return JSON-parseable string.

webrtc/stats/rtcstatsreport.cc

Index: webrtc/stats/rtcstatsreport.cc
diff --git a/webrtc/stats/rtcstatsreport.cc b/webrtc/stats/rtcstatsreport.cc
index 566aef8772de09d18103123d8a9a17082e523bac..f6736d1f91a93ce6cfbfb0e3faa4dfb8ceb7c45d 100644
--- a/webrtc/stats/rtcstatsreport.cc
+++ b/webrtc/stats/rtcstatsreport.cc
@@ -107,10 +107,11 @@ std::string RTCStatsReport::ToString() const {
   std::ostringstream oss;
   ConstIterator it = begin();
   if (it != end()) {
-    oss << it->ToString();
+    oss << '[' << it->ToString();
     for (++it; it != end(); ++it) {
-      oss << '\n' << it->ToString();
+      oss << ",\n" << it->ToString();
     }
+    oss << ']';

[hbos, 2017/07/25 12:29:21]

Is it valid JSON to have just an array, and not a surrounding dictionary? Will
this be used for JSON parsing?

If it needs to be updated note that the stats report is a Map between stats IDs
and stats objects. Also note that this might affect the desired indentation of
the output string, if we still want to split it up into multiple lines.

Also note: the report also has a timestamp which currently is not included, but
it isn't in the spec so feel free to keep skipping it.

   }
   return oss.str();
 }