src: expose TraceEventHelper with NODE_EXTERN

As node requires a tracing controller to be initialized embedders need
access to the TraceEventHelper so that we can actually set the tracing
controller.

Refs: https://github.com/electron/electron/commit/0e5b6f93000e4718c9e35332ddbd0f6b76cdd585/#diff-89b287b2edd0a02dddae60cb26157f47

PR-URL: #28724
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: MarshallOfSound

src/tracing/trace_event.h

@@ -310,7 +310,9 @@ const int kZeroNumArgs = 0;
 const decltype(nullptr) kGlobalScope = nullptr;
 const uint64_t kNoId = 0;
 
-class TraceEventHelper {
+// Extern (for now) because embedders need access to TraceEventHelper.
+// Refs: https://github.com/nodejs/node/pull/28724
+class NODE_EXTERN TraceEventHelper {
  public:
   static TracingController* GetTracingController();
   static Agent* GetAgent();