Use the task scheduler in the new Windows USB backend

device/usb/usb_service_win.h

 // Copyright 2017 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 "device/usb/usb_service.h"
 
 #include <list>
 #include <unordered_map>
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/scoped_observer.h"
 #include "device/base/device_monitor_win.h"
 #include "device/usb/usb_device_win.h"
 
-namespace base {
-class SequencedTaskRunner;
-}
-
 namespace device {
 
 class UsbServiceWin : public DeviceMonitorWin::Observer, public UsbService {
  public:
-  explicit UsbServiceWin(
-      scoped_refptr<base::SequencedTaskRunner> blocking_task_runner);
+  UsbServiceWin();
   ~UsbServiceWin() override;
 
  private:
-  class BlockingThreadHelper;
+  class BlockingTaskHelper;
 
   // device::UsbService implementation
+  void Shutdown() override;
   void GetDevices(const GetDevicesCallback& callback) override;
 
   // device::DeviceMonitorWin::Observer implementation
   void OnDeviceAdded(const GUID& class_guid,
                      const std::string& device_path) override;
   void OnDeviceRemoved(const GUID& class_guid,
                        const std::string& device_path) override;
 
   // Methods called by BlockingThreadHelper
   void HelperStarted();
   void CreateDeviceObject(const std::string& device_path,
                           const std::string& hub_path,
                           int port_number,
                           const std::string& driver_name);
 
   void DeviceReady(scoped_refptr<UsbDeviceWin> device, bool success);
 
   bool enumeration_ready() {
     return helper_started_ && first_enumeration_countdown_ == 0;
   }
 
   // Enumeration callbacks are queued until an enumeration completes.
   bool helper_started_ = false;
   uint32_t first_enumeration_countdown_ = 0;
   std::list<GetDevicesCallback> enumeration_callbacks_;
 
-  BlockingThreadHelper* helper_;
+  std::unique_ptr<BlockingTaskHelper> helper_;
   std::unordered_map<std::string, scoped_refptr<UsbDeviceWin>> devices_by_path_;
 
   ScopedObserver<DeviceMonitorWin, DeviceMonitorWin::Observer> device_observer_;
 
   base::WeakPtrFactory<UsbServiceWin> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(UsbServiceWin);
 };
 
 }  // namespace device