[swarming] Isolate custom libcxx.

build/isolate.gypi

 # Copyright 2015 the V8 project authors. All rights reserved.
 # Copyright (c) 2012 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.
 
 # This file is meant to be included into a target to provide a rule
 # to "build" .isolate files into a .isolated file.
 #
 # To use this, create a gyp target with the following form:
 # 'conditions': [
@@ skipping 52 matching lines @@
         # for more details.
 
         # Path variables are used to replace file paths when loading a .isolate
         # file
         '--path-variable', 'DEPTH', '<(DEPTH)',
         '--path-variable', 'PRODUCT_DIR', '<(PRODUCT_DIR)',
 
         '--config-variable', 'OS=<(OS)',
         '--config-variable', 'component=<(component)',
         '--config-variable', 'target_arch=<(target_arch)',
+        '--config-variable', 'use_custom_libcxx=<(use_custom_libcxx)',
         '--config-variable', 'v8_use_external_startup_data=<(v8_use_external_startup_data)',
         '--config-variable', 'v8_use_snapshot=<(v8_use_snapshot)',
       ],
     },
   ],
 }