Add libc++ and libc++abi to third-party.

base/base_untrusted.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'includes': [
     '../build/common_untrusted.gypi',
@@ skipping 16 matching lines @@
               'base_switches.cc',
               'base_switches.h',
               'strings/string16.cc',
               'sync_socket_nacl.cc',
               'time/time_posix.cc',
             ],
           },
           'dependencies': [
             '<(DEPTH)/native_client/tools.gyp:prep_toolchain',
           ],
+          'conditions': [
+            ['use_libcxx==1',{
+              'dependencies!': [
+                '<(DEPTH)/third_party/libc++/libc++.gyp:libc++',
+                '<(DEPTH)/third_party/libc++abi/libc++abi.gyp:libc++abi',
+              ],
+            }],
+          ],
         },
       ],
     }],
   ],
 }