Fix BM_ShellShutdown regression (#18676)

shell/common/shell_benchmarks.cc

@@ -66,6 +66,17 @@ static void StartupAndShutdownShell(benchmark::State& state,
 
   FML_CHECK(shell);
 
+  {
+    // The ui thread could be busy processing tasks after shell created, e.g.,
+    // default font manager setup. The measurement of shell shutdown should be
+    // considered after those ui tasks have been done.
+    benchmarking::ScopedPauseTiming pause(state, true);
+    fml::AutoResetWaitableEvent latch;
+    fml::TaskRunner::RunNowOrPostTask(thread_host->ui_thread->GetTaskRunner(),
+                                      [&latch]() { latch.Signal(); });
+    latch.Wait();
+  }
+
   {
     benchmarking::ScopedPauseTiming pause(state, !measure_shutdown);
     // Shutdown must occur synchronously on the platform thread.