[Tracing] Use TracingCategoryObserver in runtime statistics

src/tracing/trace-event.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/tracing/trace-event.h"
 
 #include <string.h>
 
 #include "src/counters.h"
 #include "src/isolate.h"
 #include "src/tracing/traced-value.h"
 #include "src/v8.h"
 
 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();
 }
 
 void CallStatsScopedTracer::AddEndTraceEvent() {
   if (!has_parent_scope_ && p_data_->isolate) {
     auto value = v8::tracing::TracedValue::Create();
     p_data_->isolate->counters()->runtime_call_stats()->Dump(value.get());
     v8::internal::tracing::AddTraceEvent(
         TRACE_EVENT_PHASE_END, p_data_->category_group_enabled, p_data_->name,
@@ skipping 20 matching lines @@
   if (!has_parent_scope_ && table->current_timer() == NULL) table->Reset();
   v8::internal::tracing::AddTraceEvent(
       TRACE_EVENT_PHASE_BEGIN, category_group_enabled, name,
       v8::internal::tracing::kGlobalScope, v8::internal::tracing::kNoId,
       TRACE_EVENT_FLAG_NONE, v8::internal::tracing::kNoId);
 }
 
 }  // namespace tracing
 }  // namespace internal
 }  // namespace v8