[Tracing] Implement TracingCategoryObserver.

include/v8-platform.h

 // Copyright 2013 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_PLATFORM_H_
 #define V8_V8_PLATFORM_H_
 
 #include <stddef.h>
 #include <stdint.h>
 #include <memory>
@@ skipping 189 matching lines @@
   virtual void UpdateTraceEventDuration(const uint8_t* category_enabled_flag,
                                         const char* name, uint64_t handle) {}
 
   class TraceStateObserver {
    public:
     virtual ~TraceStateObserver() = default;
     virtual void OnTraceEnabled() = 0;
     virtual void OnTraceDisabled() = 0;
   };
 
+  class TracingFlag {
+   public:
+    enum Mode {
+      ENABLED_BY_NATIVE = 1 << 0,
+      ENABLED_BY_TRACING = 1 << 1,
+    };
+    static std::unique_ptr<TracingFlag> Create();
+    virtual ~TracingFlag() = default;
+
+   protected:
+    TracingFlag() = default;
+  };
+
   /** Adds tracing state change observer. */
   virtual void AddTraceStateObserver(TraceStateObserver*) {}
 
   /** Removes tracing state change observer. */
   virtual void RemoveTraceStateObserver(TraceStateObserver*) {}
 };
 
 }  // namespace v8
 
 #endif  // V8_V8_PLATFORM_H_