[Tracing] Use TracingCategoryObserver in gc statistics

src/tracing/tracing-category-observer.cc

 // Copyright 2016 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/tracing-category-observer.h"
 
 #include "src/flags.h"
 #include "src/tracing/trace-event.h"
 #include "src/v8.h"
 
 namespace v8 {
 namespace tracing {
 
 TracingCategoryObserver* TracingCategoryObserver::instance_ = nullptr;
 
 void TracingCategoryObserver::SetUp() {
   TracingCategoryObserver::instance_ = new TracingCategoryObserver();
   v8::internal::V8::GetCurrentPlatform()->AddTraceStateObserver(
       TracingCategoryObserver::instance_);
   TRACE_EVENT_WARMUP_CATEGORY(TRACE_DISABLED_BY_DEFAULT("v8.runtime_stats"));
   TRACE_EVENT_WARMUP_CATEGORY(
       TRACE_DISABLED_BY_DEFAULT("v8.runtime_stats_sampling"));
+  TRACE_EVENT_WARMUP_CATEGORY(TRACE_DISABLED_BY_DEFAULT("v8.gc_stats"));
 }
 
 void TracingCategoryObserver::TearDown() {
   v8::internal::V8::GetCurrentPlatform()->RemoveTraceStateObserver(
       TracingCategoryObserver::instance_);
   delete TracingCategoryObserver::instance_;
 }
 
 void TracingCategoryObserver::OnTraceEnabled() {
   bool enabled = false;
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(
       TRACE_DISABLED_BY_DEFAULT("v8.runtime_stats"), &enabled);
   if (enabled) {
     v8::internal::FLAG_runtime_stats |= ENABLED_BY_TRACING;
   }
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(
       TRACE_DISABLED_BY_DEFAULT("v8.runtime_stats_sampling"), &enabled);
   if (enabled) {
     v8::internal::FLAG_runtime_stats |= ENABLED_BY_SAMPLING;
   }
+  TRACE_EVENT_CATEGORY_GROUP_ENABLED(TRACE_DISABLED_BY_DEFAULT("v8.gc_stats"),
+                                     &enabled);
+  if (enabled) {
+    v8::internal::FLAG_gc_stats |= ENABLED_BY_TRACING;
+  }
 }
 
 void TracingCategoryObserver::OnTraceDisabled() {
   v8::internal::FLAG_runtime_stats &=
       ~(ENABLED_BY_TRACING | ENABLED_BY_SAMPLING);
+  v8::internal::FLAG_gc_stats &= ~ENABLED_BY_TRACING;
 }
 
 }  // namespace tracing
 }  // namespace v8