[tracing] Implement Add/RemoveTraceStateObserver for default platform.

src/libplatform/default-platform.h

Index: src/libplatform/default-platform.h
diff --git a/src/libplatform/default-platform.h b/src/libplatform/default-platform.h
index 0fd7e5ad890a3090ab794c9d5c734a6ea29f45f3..4a6f6f2821a94dff3f93786bad3ba7f46545c479 100644
--- a/src/libplatform/default-platform.h
+++ b/src/libplatform/default-platform.h
@@ -61,6 +61,9 @@ class DefaultPlatform : public Platform {
                                 const char* name, uint64_t handle) override;
   void SetTracingController(tracing::TracingController* tracing_controller);
 
+  void AddTraceStateObserver(TraceStateObserver* observer) override;
+  void RemoveTraceStateObserver(TraceStateObserver* observer) override;
+
  private:
   static const int kMaxThreadPoolSize;