[Tracing] Implement TracingCategoryObserver.

include/v8-platform.h

Index: include/v8-platform.h
diff --git a/include/v8-platform.h b/include/v8-platform.h
index e11567488b6a3ca45780d52d11d6d1b3696b44ba..01c110d6b42f43680b674134864c9a5980970df8 100644
--- a/include/v8-platform.h
+++ b/include/v8-platform.h
@@ -207,6 +207,19 @@ class Platform {
     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*) {}