[Tracing] Use TracingCategoryObserver in runtime 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 "include/v8.h"
 #include "src/flags.h"
 #include "src/tracing/trace-event.h"
 #include "src/v8.h"
 
 namespace v8 {
 namespace tracing {
 
 std::unique_ptr<TracingCategoryObserver> TracingCategoryObserver::Create() {
   return std::unique_ptr<TracingCategoryObserver>(
       new TracingCategoryObserverImpl());
 }
 
-TracingCategoryObserverImpl::TracingCategoryObserverImpl() {}
+TracingCategoryObserverImpl::TracingCategoryObserverImpl() {
+  TRACE_EVENT_WARMUP_CATEGORY(TRACE_DISABLED_BY_DEFAULT("v8.runtime_stats"));
+}
 
 TracingCategoryObserverImpl::~TracingCategoryObserverImpl() {
   OnTraceDisabled();
 }
 
 void TracingCategoryObserverImpl::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;
   }
 }
 
 void TracingCategoryObserverImpl::OnTraceDisabled() {
-  v8::internal::FLAG_runtime_stats &= ~ENABLED_BY_TRACING;
+  v8::internal::FLAG_runtime_stats ^= ENABLED_BY_TRACING;

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

This won't work. The category might not have been enabled before.

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

Done.

ah you are right, I think the original one will work.

 }
 
 }  // namespace tracing
 }  // namespace v8