Add a feature flag for the new Windows USB backend.

device/usb/usb_service_win.cc

Index: device/usb/usb_service_win.cc
diff --git a/device/usb/usb_service_win.cc b/device/usb/usb_service_win.cc
new file mode 100644
index 0000000000000000000000000000000000000000..6feea8913d1652e068e91d23408273b213ffbfa5
--- /dev/null
+++ b/device/usb/usb_service_win.cc
@@ -0,0 +1,15 @@
+// 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_win.h"
+
+namespace device {
+
+UsbServiceWin::UsbServiceWin(
+    scoped_refptr<base::SequencedTaskRunner> blocking_task_runner)
+    : UsbService(blocking_task_runner) {}
+
+UsbServiceWin::~UsbServiceWin() {}
+
+}  // namespace device