Add a feature flag for the new Windows USB backend.

device/base/BUILD.gn

 # 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.
 
 import("//build/config/features.gni")
 
 component("base") {
   output_name = "device_base"
 
   sources = [
     "device_client.cc",
     "device_client.h",
     "device_info_query_win.cc",
     "device_info_query_win.h",
     "device_monitor_win.cc",
     "device_monitor_win.h",
+    "features.cc",
+    "features.h",
   ]
 
   defines = [ "DEVICE_BASE_IMPLEMENTATION" ]
 
   deps = []
 
   public_deps = [
     "//base",
   ]
 
@@ skipping 18 matching lines @@
     "mock_device_client.cc",
     "mock_device_client.h",
   ]
 
   deps = [
     ":base",
     "//device/hid:mocks",
     "//device/usb:test_support",
   ]
 }