[tracing] Implement Add/RemoveTraceStateObserver for default platform.

include/libplatform/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_LIBPLATFORM_V8_TRACING_H_
 #define V8_LIBPLATFORM_V8_TRACING_H_
 
 #include <fstream>
 #include <memory>
+#include <unordered_set>
 #include <vector>
 
+#include "include/v8-platform.h"
+
 namespace v8 {
+
+namespace base {
+class Mutex;
+}  // namespace base
+
 namespace platform {
 namespace tracing {
 
 const int kTraceMaxNumArgs = 2;
 
 class TraceObject {
  public:
   union ArgValue {
     bool as_bool;
     uint64_t as_uint;
@@ skipping 187 matching lines @@
   // These values must be in sync with macro values in TraceEvent.h in Blink.
   enum CategoryGroupEnabledFlags {
     // Category group enabled for the recording mode.
     ENABLED_FOR_RECORDING = 1 << 0,
     // Category group enabled by SetEventCallbackEnabled().
     ENABLED_FOR_EVENT_CALLBACK = 1 << 2,
     // Category group enabled to export events to ETW.
     ENABLED_FOR_ETW_EXPORT = 1 << 3
   };
 
-  TracingController() {}
+  TracingController();
+  ~TracingController();
   void Initialize(TraceBuffer* trace_buffer);
   const uint8_t* GetCategoryGroupEnabled(const char* category_group);
   static const char* GetCategoryGroupName(const uint8_t* category_enabled_flag);
   uint64_t AddTraceEvent(char phase, const uint8_t* category_enabled_flag,
                          const char* name, const char* scope, uint64_t id,
                          uint64_t bind_id, int32_t num_args,
                          const char** arg_names, const uint8_t* arg_types,
                          const uint64_t* arg_values, unsigned int flags);
   void UpdateTraceEventDuration(const uint8_t* category_enabled_flag,
                                 const char* name, uint64_t handle);
 
   void StartTracing(TraceConfig* trace_config);
   void StopTracing();
 
+  void AddTraceStateObserver(Platform::TraceStateObserver* observer);
+  void RemoveTraceStateObserver(Platform::TraceStateObserver* observer);
+
  private:
   const uint8_t* GetCategoryGroupEnabledInternal(const char* category_group);
   void UpdateCategoryGroupEnabledFlag(size_t category_index);
   void UpdateCategoryGroupEnabledFlags();
 
   std::unique_ptr<TraceBuffer> trace_buffer_;
   std::unique_ptr<TraceConfig> trace_config_;
+  std::unique_ptr<base::Mutex> mutex_;
+  std::unordered_set<Platform::TraceStateObserver*> observers_;
   Mode mode_ = DISABLED;
 
   // Disallow copy and assign
   TracingController(const TracingController&) = delete;
   void operator=(const TracingController&) = delete;
 };
 
 }  // namespace tracing
 }  // namespace platform
 }  // namespace v8
 
 #endif  // V8_LIBPLATFORM_V8_TRACING_H_