[Tracing] Implement TracingCategoryObserver.

src/tracing/tracing-flag.h

+// 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.
+
+#ifndef V8_TRACING_TRACING_FLAG_H_
+#define V8_TRACING_TRACING_FLAG_H_
+
+#include "include/v8-platform.h"
+
+namespace v8 {
+namespace tracing {
+
+class TracingFlagImpl : public Platform::TracingFlag,
+                        private Platform::TraceStateObserver {
+ public:
+  TracingFlagImpl();
+  ~TracingFlagImpl();
+
+  // v8::Platform::TraceStateObserver
+  void OnTraceEnabled() final;
+  void OnTraceDisabled() final;
+
+ private:
+  v8::Platform* platform_;
+};
+
+}  // namespace tracing
+}  // namespace v8
+#endif  // V8_TRACING_TRACING_FLAG_H_