[tracing] Implement Add/RemoveTraceStateObserver for default platform.

src/libplatform/default-platform.cc

 // 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.
 
 #include "src/libplatform/default-platform.h"
 
 #include <algorithm>
 #include <queue>
 
 #include "src/base/logging.h"
@@ skipping 207 matching lines @@
 
 void DefaultPlatform::SetTracingController(
     tracing::TracingController* tracing_controller) {
   tracing_controller_ = tracing_controller;
 }
 
 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