[tracing] Implement Add/RemoveTraceStateObserver for default platform.

src/libplatform/default-platform.cc

Index: src/libplatform/default-platform.cc
diff --git a/src/libplatform/default-platform.cc b/src/libplatform/default-platform.cc
index 2f81248ec189cefddeeb3ba7d12b898fdecdc7c4..ba7d0851cac541905b5d6d77eec35b92c9f2604d 100644
--- a/src/libplatform/default-platform.cc
+++ b/src/libplatform/default-platform.cc
@@ -225,5 +225,15 @@ size_t DefaultPlatform::NumberOfAvailableBackgroundThreads() {
   return static_cast<size_t>(thread_pool_size_);
 }
 
+void DefaultPlatform::AddTraceStateObserver(TraceStateObserver* observer) {
+  if (!tracing_controller_) return;
+  tracing_controller_->AddTraceStateObserver(observer);
+}
+
+void DefaultPlatform::RemoveTraceStateObserver(TraceStateObserver* observer) {
+  if (!tracing_controller_) return;
+  tracing_controller_->RemoveTraceStateObserver(observer);
+}
+
 }  // namespace platform
 }  // namespace v8