Use the task scheduler in the new Windows USB backend

device/usb/usb_service_win.h

Index: device/usb/usb_service_win.h
diff --git a/device/usb/usb_service_win.h b/device/usb/usb_service_win.h
index 58f4bb2bc8961d8271a4412cfaf0e9e9e66c47e5..8a9d7ecabeae8aa6705e0fd550b7e4ed8990ee51 100644
--- a/device/usb/usb_service_win.h
+++ b/device/usb/usb_service_win.h
@@ -13,22 +13,18 @@
 #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
@@ -55,7 +51,7 @@ class UsbServiceWin : public DeviceMonitorWin::Observer, public UsbService {
   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_;