[tracing] Support ConvertableToTraceFormat argument type.

src/libplatform/tracing/trace-writer.h

 // Copyright 2016 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.
 
 #ifndef SRC_LIBPLATFORM_TRACING_TRACE_WRITER_H_
 #define SRC_LIBPLATFORM_TRACING_TRACE_WRITER_H_
 
 #include "include/libplatform/v8-tracing.h"
 
 namespace v8 {
 namespace platform {
 namespace tracing {
 
 class JSONTraceWriter : public TraceWriter {
  public:
   explicit JSONTraceWriter(std::ostream& stream);
   ~JSONTraceWriter();
   void AppendTraceEvent(TraceObject* trace_event) override;
   void Flush() override;
 
  private:
   void AppendArgValue(uint8_t type, TraceObject::ArgValue value);
+  void AppendArgValue(v8::ConvertableToTraceFormat*);
 
   std::ostream& stream_;
   bool append_comma_ = false;
 };
 
 }  // namespace tracing
 }  // namespace platform
 }  // namespace v8
 
 #endif  // SRC_LIBPLATFORM_TRACING_TRACE_WRITER_H_