Fix Windows GN build.

build/config/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.
 
 config("my_msvs") {
   defines = [
       "CHROMIUM_BUILD",
       "TOOLKIT_VIEWS=1",
       "USE_LIBJPEG_TURBO=1",
       "ENABLE_ONE_CLICK_SIGNIN",
@@ skipping 13 matching lines @@
       "ENABLE_BACKGROUND=1",
       "ENABLE_AUTOMATION=1",
       "ENABLE_GOOGLE_NOW=1",
       "ENABLE_PRINTING=1",
       "ENABLE_CAPTIVE_PORTAL_DETECTION=1",
       "ENABLE_APP_LIST=1",
       "ENABLE_MESSAGE_CENTER=1",
       "ENABLE_SETTINGS_APP=1",
       "ENABLE_MANAGED_USERS=1",
   ]
+
+  if (use_aura) {
+    defines += [ "USE_AURA=1" ]
+  }
 }
 
 config("feature_flags") {
   #defines = 
 }
 
 config("debug") {
   defines = [
     "_DEBUG",
     "DYNAMIC_ANNOTATIONS_ENABLED=1",
     "WTF_USE_DYNAMIC_ANNOTATIONS=1",
   ]
 
   if (is_linux && !is_android && cpu_arch == "x64") {
     # Enable libstdc++ debugging facilities to help catch problems early, see
     # http://crbug.com/65151 .
     # TODO(phajdan.jr): Should we enable this for all of POSIX?
     defines += [ "_GLIBCXX_DEBUG=1" ]
   }
 }
 
 config("release") {
 
 }