[Tracing] Implement TracingCategoryObserver.

src/tracing/tracing-category-observer.cc

Index: src/tracing/tracing-category-observer.cc
diff --git a/src/tracing/tracing-category-observer.cc b/src/tracing/tracing-category-observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..f9315b4ce456beb5d55b0b02c6ca06d00e6b006e
--- /dev/null
+++ b/src/tracing/tracing-category-observer.cc
@@ -0,0 +1,40 @@
+// 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() {
+  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;
+}
+
+}  // namespace tracing
+}  // namespace v8