[Tracing] Use TracingCategoryObserver in runtime statistics

src/tracing/trace-event.cc

Index: src/tracing/trace-event.cc
diff --git a/src/tracing/trace-event.cc b/src/tracing/trace-event.cc
index ebee5345997756c3a77fc902eac3089de7b16369..dd7ce71a6dfb728e6addd951efa8ce2d18538ac5 100644
--- a/src/tracing/trace-event.cc
+++ b/src/tracing/trace-event.cc
@@ -15,10 +15,6 @@ namespace v8 {
 namespace internal {
 namespace tracing {
 
-// A global flag used as a shortcut to check for the
-// v8.runtime-call-stats category due to its high frequency use.
-base::Atomic32 kRuntimeCallStatsTracingEnabled = false;
-
 v8::Platform* TraceEventHelper::GetCurrentPlatform() {
   return v8::internal::V8::GetCurrentPlatform();
 }