[Tracing w/SDK] Call Flush before tracing stop

The asynchronous stop requires us to call Flush() explicitly because
no other flushes will happen before the stop signal.

Change-Id: I09f189f79c9e5316b0cfd5b7500aaf5a62a15aec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3955714
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Mikhail Khokhlov <khokhlov@google.com>
Cr-Commit-Position: refs/heads/main@{#1059880}

services/tracing/public/cpp/perfetto/custom_event_recorder.cc

@@ -227,6 +227,9 @@ void CustomEventRecorder::OnTracingStopped(
   }
 
 #if BUILDFLAG(USE_PERFETTO_CLIENT_LIBRARY)
+  // We have to flush explicitly because we're using the asynchronous stop
+  // mechanism.
+  perfetto::TrackEvent::Flush();
   std::move(stop_complete_callback).Run();
 #endif  // BUILDFLAG(USE_PERFETTO_CLIENT_LIBRARY)
 }