[Tracing] Expose trace serialization as part of tracing interface

src/libplatform/tracing/trace-writer.h

Index: src/libplatform/tracing/trace-writer.h
diff --git a/src/libplatform/tracing/trace-writer.h b/src/libplatform/tracing/trace-writer.h
index 963fc6a64dc262172db2949cc4f4f568d4fae8b1..3a3d4f08a985c2185768f6cff790308477221042 100644
--- a/src/libplatform/tracing/trace-writer.h
+++ b/src/libplatform/tracing/trace-writer.h
@@ -19,10 +19,7 @@ class JSONTraceWriter : public TraceWriter {
   void Flush() override;
 
  private:
-  void AppendArgValue(uint8_t type, TraceObject::ArgValue value);
-
-  std::ostream& stream_;
-  bool append_comma_ = false;
+  TraceSerializer trace_serializer_;
 };
 
 }  // namespace tracing