Add a feature flag for the new Windows USB backend.

device/usb/mock_usb_service.cc

 // 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.
 
 #include "device/usb/mock_usb_service.h"
 
 #include <string>
 #include <vector>
 
 #include "base/threading/thread_task_runner_handle.h"
 #include "device/usb/usb_device.h"
 
 namespace device {
 
-MockUsbService::MockUsbService()
-    : UsbService(base::ThreadTaskRunnerHandle::IsSet()
-                     ? base::ThreadTaskRunnerHandle::Get()
-                     : nullptr,
-                 nullptr) {}
+MockUsbService::MockUsbService() : UsbService(nullptr) {}
 
 MockUsbService::~MockUsbService() {
   // Shutdown() must be called before the base class destructor.
   Shutdown();
 }
 
 void MockUsbService::AddDevice(scoped_refptr<UsbDevice> device) {
   devices()[device->guid()] = device;
   NotifyDeviceAdded(device);
 }
 
 void MockUsbService::RemoveDevice(scoped_refptr<UsbDevice> device) {
   devices().erase(device->guid());
   UsbService::NotifyDeviceRemoved(device);
 }
 
 }  // namespace device