Add a feature flag for 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
new file mode 100644
index 0000000000000000000000000000000000000000..a6feb81a3e02d4e885cbb4cf48c073423a2ce95d
--- /dev/null
+++ b/device/usb/usb_service_win.h
@@ -0,0 +1,21 @@
+// 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 "base/macros.h"
+
+namespace device {
+
+class UsbServiceWin : public UsbService {
+ public:
+  explicit UsbServiceWin(
+      scoped_refptr<base::SequencedTaskRunner> blocking_task_runner);
+  ~UsbServiceWin() override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(UsbServiceWin);
+};
+
+}  // namespace device