[tracing] Support ConvertableToTraceFormat argument type.

src/libplatform/tracing/trace-writer.cc

Index: src/libplatform/tracing/trace-writer.cc
diff --git a/src/libplatform/tracing/trace-writer.cc b/src/libplatform/tracing/trace-writer.cc
index ec95527d5f6a892e4cc9437b5f526af471219830..4d08c967df2091a122fd79728ab6fe786a7b36e7 100644
--- a/src/libplatform/tracing/trace-writer.cc
+++ b/src/libplatform/tracing/trace-writer.cc
@@ -7,6 +7,7 @@
 #include <cmath>
 
 #include "base/trace_event/common/trace_event_common.h"
+#include "include/v8-platform.h"
 #include "src/base/platform/platform.h"
 
 namespace v8 {
@@ -106,6 +107,15 @@ void JSONTraceWriter::AppendArgValue(uint8_t type,
         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;