Add a feature flag for the new Windows USB backend.

device/base/features.h

+// 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.
+
+#ifndef DEVICE_BASE_FEATURES_H_
+#define DEVICE_BASE_FEATURES_H_
+
+#include "base/feature_list.h"
+#include "build/build_config.h"
+#include "device/base/device_base_export.h"
+
+namespace device {
+
+#if defined(OS_WIN)
+DEVICE_BASE_EXPORT extern const base::Feature kNewUsbBackend;
+#endif  // defined(OS_WIN)
+
+}  // namespace device
+
+#endif  // DEVICE_BASE_FEATURES_H_