[Tracing] Use TracingCategoryObserver in runtime statistics

src/counters.cc

Index: src/counters.cc
diff --git a/src/counters.cc b/src/counters.cc
index ec66d885fda70fcd119cfa46f3e542769e17f93f..c7aa0e9dc46f2a09d4088ab7f963167d09d47156 100644
--- a/src/counters.cc
+++ b/src/counters.cc
@@ -344,9 +344,9 @@ void RuntimeCallStats::Print(std::ostream& os) {
 }
 
 void RuntimeCallStats::Reset() {
-  if (!FLAG_runtime_call_stats &&
-      !TRACE_EVENT_RUNTIME_CALL_STATS_TRACING_ENABLED())
+  if (V8_LIKELY(FLAG_runtime_stats == 0)) {

[alph, 2016/10/31 20:46:53]

nit: no need {} for return;

[lpy, 2016/10/31 20:54:19]

I think Yang once mentioned V8 prefers {}

[alph, 2016/10/31 21:17:19]

Interesting. the code style guide
https://google.github.io/styleguide/cppguide.html states that you don't need {}
for single-line statements

     return;
+  }
 #define RESET_COUNTER(name) this->name.Reset();
   FOR_EACH_MANUAL_COUNTER(RESET_COUNTER)
 #undef RESET_COUNTER