[Tracing] Embed V8 runtime call stats into tracing.

src/counters.cc

Index: src/counters.cc
diff --git a/src/counters.cc b/src/counters.cc
index 57dad3db1d9a22409a163d3218794b384180a044..829c94d9568bc2fd05f0ec553492abeeda88f577 100644
--- a/src/counters.cc
+++ b/src/counters.cc
@@ -273,6 +273,11 @@ void RuntimeCallCounter::Reset() {
   time = base::TimeDelta();
 }
 
+void RuntimeCallCounter::Dump(std::stringstream& out) {
+  out << "\"" << name << "\":[" << count << "," << time.InMicroseconds()
+      << "],";
+}
+
 // static
 void RuntimeCallStats::Enter(Isolate* isolate, RuntimeCallTimer* timer,
                              CounterId counter_id) {