[engine] always force platform channel responses to schedule a task. (#54975)

If we use runNowOrPostTask on platform channel responses, then we may not wake up the dart message loop. If nothing else wakes it up, then the embedder may hang on platform channel responses.

This fixes several google3 integration tests when run in merged thread mode.

Author: Jonah Williams

shell/common/shell.cc

@@ -1076,10 +1076,10 @@ void Shell::OnPlatformViewDispatchPlatformMessage(
 
   // The static leak checker gets confused by the use of fml::MakeCopyable.
   // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks)
-  fml::TaskRunner::RunNowOrPostTask(
-      task_runners_.GetUITaskRunner(),
-      fml::MakeCopyable([engine = engine_->GetWeakPtr(),
-                         message = std::move(message)]() mutable {
+  // This logic must always explicitly post a task so that we are guaranteed
+  // to wake up the UI message loop to flush tasks.
+  task_runners_.GetUITaskRunner()->PostTask(fml::MakeCopyable(
+      [engine = engine_->GetWeakPtr(), message = std::move(message)]() mutable {
         if (engine) {
           engine->DispatchPlatformMessage(std::move(message));
         }