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

build/common_untrusted.gypi

 # 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 GYP file should be included for every target in Chromium that is built
 # using the NaCl toolchain.
 {
   'includes': [
     '../native_client/build/untrusted.gypi',
   ],
   'target_defaults': {
     'conditions': [
       ['target_arch=="arm"', {
         'variables': {
           'clang': 1,
         },
         'defines': [
           # Needed by build/build_config.h processor architecture detection.
           '__ARMEL__',
           # Needed by base/third_party/nspr/prtime.cc.
           '__arm__',
           # Disable ValGrind. The assembly code it generates causes the build
           # to fail.
           'NVALGRIND',
         ],
       }],
+      ['use_libcxx==1', {
+        'dependencies!': [
+          '<(DEPTH)/third_party/libc++/libc++.gyp:libc++',
+          '<(DEPTH)/third_party/libc++abi/libc++abi.gyp:libc++abi',
+        ],
+      }],
     ],
   },
-}
+}