Adding variable buillder for builders using win_gn folder

masters/master.tryserver.chromium.win/slaves.cfg

 # -*- python -*-
 # ex: set syntax=python:
 # 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.
 
 def distribute_slaves(slave_numbers, builders, os_ver, pool=None, gce=False):
   hostname_format = 'vm%d-m4' if not gce else 'slave%d-c4'
   return [
     {
@@ skipping 45 matching lines @@
       optional_slaves_gn, optional_builders_gn, 'win7', 'win_optional_gn')
 
   optional_slaves_chromium = [181, 185, 186, 187, 192]
   optional_builders_chromium = [
     # directory: win
     'win_chromium_compile_rel_ng',
     'win_chromium_dbg_ng',
     'win_chromium_syzyasan_rel',
     'win_chromium_variable',
     'win_optional_gpu_tests_rel',
+    # directory: win_gn
+    'win_chromium_variable_gn',
     # directory: win_nacl_sdk
     'win_nacl_sdk',
     # directory: win_nacl_sdk_build
     'win_nacl_sdk_build',
   ]
   optional_pool_chromium = distribute_slaves(
       optional_slaves_chromium,
       optional_builders_chromium,
       'win7',
       'win_optional_chromium')
@@ skipping 62 matching lines @@
       cq_pool +
       optional_pool +
       win8_pool +
       win_pgo_pool +
       experimental_pool +
       clang_win_pool +
       clang_win_gce_pool)
 
 
 slaves = windows()