[Tracing] Implement TracingCategoryObserver.

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 28ebfc05b9839327fb596ac491514e1b341fe1a9..4900c76548eb7a64b1ef3db7f709dcd7ef0c4750 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -148,6 +148,7 @@ class PredictablePlatform : public Platform {
 
 
 v8::Platform* g_platform = NULL;
+std::unique_ptr<Platform::TracingFlag> g_tracing_flag;
 
 
 static Local<Value> Throw(Isolate* isolate, const char* message) {
@@ -2743,11 +2744,12 @@ int Shell::Main(int argc, char* argv[]) {
         trace_config =
             platform::tracing::TraceConfig::CreateDefaultTraceConfig();
       }
-      tracing_controller->Initialize(trace_buffer);
-      tracing_controller->StartTracing(trace_config);
       if (!i::FLAG_verify_predictable) {
         platform::SetTracingController(g_platform, tracing_controller);
       }
+      tracing_controller->Initialize(trace_buffer);
+      g_tracing_flag = Platform::TracingFlag::Create();
+      tracing_controller->StartTracing(trace_config);
     }
 
     if (options.dump_heap_constants) {