[tracing] Support ConvertableToTraceFormat argument type.

src/libplatform/tracing/trace-writer.cc

 // 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.
 
 #include "src/libplatform/tracing/trace-writer.h"
 
 #include <cmath>
 
 #include "base/trace_event/common/trace_event_common.h"
+#include "include/v8-platform.h"
 #include "src/base/platform/platform.h"
 
 namespace v8 {
 namespace platform {
 namespace tracing {
 
 // Writes the given string to a stream, taking care to escape characters
 // when necessary.
 V8_INLINE static void WriteJSONStringToStream(const char* str,
                                               std::ostream& stream) {
@@ skipping 79 matching lines @@
       stream_ << "\"" << value.as_pointer << "\"";
       break;
     case TRACE_VALUE_TYPE_STRING:
     case TRACE_VALUE_TYPE_COPY_STRING:
       if (value.as_string == nullptr) {
         stream_ << "\"NULL\"";
       } else {
         WriteJSONStringToStream(value.as_string, stream_);
       }
       break;
+    case TRACE_VALUE_TYPE_CONVERTABLE: {
+      std::unique_ptr<v8::ConvertableToTraceFormat> convertable_arg(
+          reinterpret_cast<v8::ConvertableToTraceFormat*>(
+              static_cast<intptr_t>(value.as_uint)));
+      std::string arg_stringified;
+      convertable_arg->AppendAsTraceFormat(&arg_stringified);
+      stream_ << arg_stringified;
+      break;
+    }
     default:
       UNREACHABLE();
       break;
   }
 }
 
 JSONTraceWriter::JSONTraceWriter(std::ostream& stream) : stream_(stream) {
   stream_ << "{\"traceEvents\":[";
 }
 
@@ skipping 35 matching lines @@
 
 void JSONTraceWriter::Flush() {}
 
 TraceWriter* TraceWriter::CreateJSONTraceWriter(std::ostream& stream) {
   return new JSONTraceWriter(stream);
 }
 
 }  // namespace tracing
 }  // namespace platform
 }  // namespace v8