Add a feature flag for the new Windows USB backend.

device/usb/BUILD.gn

 # Copyright (c) 2013 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")
 import("//testing/libfuzzer/fuzzer_test.gni")
 
 assert(!is_ios)
 
 if (is_android) {
@@ skipping 26 matching lines @@
     "usb_endpoint_android.cc",
     "usb_endpoint_android.h",
     "usb_ids.cc",
     "usb_ids.h",
     "usb_interface_android.cc",
     "usb_interface_android.h",
     "usb_service.cc",
     "usb_service.h",
     "usb_service_android.cc",
     "usb_service_android.h",
+    "usb_service_win.cc",
+    "usb_service_win.h",
     "webusb_descriptors.cc",
     "webusb_descriptors.h",
     generated_ids,
   ]
 
   deps = [
     ":usb_device_ids",
     "//base",
     "//base/third_party/dynamic_annotations",
     "//components/device_event_log",
@@ skipping 146 matching lines @@
     jni_package = "device"
   }
 
   android_library("java") {
     java_files = java_sources_needing_jni
     deps = [
       "//base:base_java",
     ]
   }
 }