fix: do not destroy thread in UI thread (#23495)

shell/browser/ui/win/dialog_thread.cc

@@ -0,0 +1,23 @@
+// Copyright (c) 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "shell/browser/ui/win/dialog_thread.h"
+
+#include "base/task/thread_pool.h"
+
+namespace dialog_thread {
+
+// Creates a SingleThreadTaskRunner to run a shell dialog on. Each dialog
+// requires its own dedicated single-threaded sequence otherwise in some
+// situations where a singleton owns a single instance of this object we can
+// have a situation where a modal dialog in one window blocks the appearance
+// of a modal dialog in another.
+TaskRunner CreateDialogTaskRunner() {
+  return base::ThreadPool::CreateCOMSTATaskRunner(
+      {base::TaskPriority::USER_BLOCKING,
+       base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN, base::MayBlock()},
+      base::SingleThreadTaskRunnerThreadMode::DEDICATED);
+}
+
+}  // namespace dialog_thread