src: use unrefed async for GC tracking

Do not let an internal handle keep the event loop alive.

PR-URL: #16758
Fixes: https://github.com/node/issues/16210
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

src/node_perf.cc

@@ -215,6 +215,7 @@ void MarkGarbageCollectionEnd(Isolate* isolate,
   uv_async_t* async = new uv_async_t();  // coverity[leaked_storage]
   if (uv_async_init(env->event_loop(), async, PerformanceGCCallback))
     return delete async;
+  uv_unref(reinterpret_cast<uv_handle_t*>(async));
   async->data =
       new PerformanceEntry::Data(env, "gc", "gc",
                                  performance_last_gc_start_mark_,

test/parallel/test-performance-gc.js

@@ -48,4 +48,6 @@ const kinds = [
   }));
   obs.observe({ entryTypes: ['gc'] });
   global.gc();
+  // Keep the event loop alive to witness the GC async callback happen.
+  setImmediate(() => setImmediate(() => 0));
 }