Add a feature flag for the new Windows USB backend.

device/usb/usb_service.h

 // Copyright 2014 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 DEVICE_USB_USB_SERVICE_H_
 #define DEVICE_USB_USB_SERVICE_H_
 
 #include <memory>
 #include <string>
 #include <unordered_map>
@@ skipping 58 matching lines @@
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
   // Methods to add and remove devices for testing purposes. Only a device added
   // by this method can be removed by RemoveDeviceForTesting().
   void AddDeviceForTesting(scoped_refptr<UsbDevice> device);
   void RemoveDeviceForTesting(const std::string& device_guid);
   void GetTestDevices(std::vector<scoped_refptr<UsbDevice>>* devices);
 
  protected:
-  UsbService(scoped_refptr<base::SingleThreadTaskRunner> task_runner,
-             scoped_refptr<base::SequencedTaskRunner> blocking_task_runner);
+  UsbService(scoped_refptr<base::SequencedTaskRunner> blocking_task_runner);
 
   void NotifyDeviceAdded(scoped_refptr<UsbDevice> device);
   void NotifyDeviceRemoved(scoped_refptr<UsbDevice> device);
 
-  scoped_refptr<base::SingleThreadTaskRunner> task_runner() {
+  const scoped_refptr<base::SingleThreadTaskRunner>& task_runner() const {
     return task_runner_;
   }
 
-  scoped_refptr<base::SequencedTaskRunner> blocking_task_runner() {
+  const scoped_refptr<base::SequencedTaskRunner>& blocking_task_runner() const {
     return blocking_task_runner_;
   }
 
   std::unordered_map<std::string, scoped_refptr<UsbDevice>>& devices() {
     return devices_;
   }
 
  private:
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
   scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_;
   std::unordered_map<std::string, scoped_refptr<UsbDevice>> devices_;
   std::unordered_set<std::string> testing_devices_;
   base::ObserverList<Observer, true> observer_list_;
 
 #if DCHECK_IS_ON()
   bool did_shutdown_ = false;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_USB_USB_SERVICE_H_