[Tracing] Make TracingCategoryObserver v8 internal.

include/v8-tracing.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_V8_TRACING_H_
-#define V8_V8_TRACING_H_
-
-#include "v8.h"  // NOLINT(build/include)
-
-namespace v8 {
-namespace tracing {
-
-class V8_EXPORT TracingCategoryObserver {
- public:
-  enum Mode {
-    ENABLED_BY_NATIVE = 1 << 0,
-    ENABLED_BY_TRACING = 1 << 1,
-  };
-  static std::unique_ptr<TracingCategoryObserver> Create();
-  virtual ~TracingCategoryObserver() = default;
-
- protected:
-  TracingCategoryObserver() = default;
-};
-
-}  // namespace tracing
-}  // namespace v8
-
-#endif  // V8_V8_TRACING_H_