[Tracing] Implement TracingCategoryObserver.

src/tracing/tracing-flag.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-flag.h"
+
+#include "include/v8.h"
+#include "src/flags.h"
+#include "src/tracing/trace-event.h"
+#include "src/v8.h"
+
+namespace v8 {
+
+std::unique_ptr<Platform::TracingFlag> Platform::TracingFlag::Create() {
+  return std::unique_ptr<Platform::TracingFlag>(new tracing::TracingFlagImpl());
+}
+
+namespace tracing {
+
+TracingFlagImpl::TracingFlagImpl()
+    : platform_(v8::internal::V8::GetCurrentPlatform()) {
+  platform_->AddTraceStateObserver(this);
+}
+
+TracingFlagImpl::~TracingFlagImpl() {
+  OnTraceDisabled();
+  platform_->RemoveTraceStateObserver(this);
+}
+
+void TracingFlagImpl::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 TracingFlagImpl::OnTraceDisabled() {
+  v8::internal::FLAG_runtime_stats &= ~ENABLED_BY_TRACING;
+}
+
+}  // namespace tracing
+}  // namespace v8