[tracing] Introduce TraceStateObserver interface to the platform.

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>
 
@@ skipping 146 matching lines @@
       const uint64_t* arg_values, unsigned int flags) {
     return 0;
   }
 
   /**
    * Sets the duration field of a COMPLETE trace event. It must be called with
    * the handle returned from AddTraceEvent().
    **/
   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;
+  };
+
+  /** 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_