src: ensure no more platform foreground tasks after Deinit

Node first calls `Isolate::Dispose`, then
`NodePlatform::UnregisterIsolate`.
This again calls `PerIsolatePlatformData::Shutdown`, which (before this
patch) called `FlushForegroundTasksInternal`, which might call
`RunForegroundTask` if it finds foreground tasks to be executed. This
will fail however, since `Isolate::GetCurrent` was already reset during
`Isolate::Dispose`.
Hence remove the check to `FlushForegroundTasksInternal` and add checks
instead that no more foreground tasks are scheduled.

Refs: v8#86

PR-URL: #25653
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: backes

src/node_platform.cc

@@ -265,7 +265,8 @@ void PerIsolatePlatformData::Shutdown() {
   if (flush_tasks_ == nullptr)
     return;
 
-  while (FlushForegroundTasksInternal()) {}
+  CHECK_NULL(foreground_delayed_tasks_.Pop());
+  CHECK_NULL(foreground_tasks_.Pop());
   CancelPendingDelayedTasks();
 
   uv_close(reinterpret_cast<uv_handle_t*>(flush_tasks_),