[Ozone-DRI] Remove the extra helper thread and reuse the IO one

ui/ozone/platform/dri/dri_helper_thread.h

-// Copyright 2015 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.
-
-#ifndef UI_OZONE_PLATFORM_DRI_DRI_HELPER_THREAD_H_
-#define UI_OZONE_PLATFORM_DRI_DRI_HELPER_THREAD_H_
-
-#include "base/threading/thread.h"
-
-namespace ui {
-
-class DriHelperThread : public base::Thread {
- public:
-  DriHelperThread();
-  ~DriHelperThread() override;
-
-  // Call to start the thread. This needs to be called after the GPU entered the
-  // sandbox.
-  void Initialize();
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(DriHelperThread);
-};
-
-}  // namespace ui
-
-#endif  // UI_OZONE_PLATFORM_DRI_DRI_HELPER_THREAD_H_