Create a dummy sandbox_unittests target on Mac.

sandbox/sandbox.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,
   },
   'conditions': [
     [ 'OS=="win"', {
       'includes': [
         'win/sandbox_win.gypi',
       ],
     }],
     [ 'OS=="linux" or OS=="android"', {
       'includes': [
         'linux/sandbox_linux.gypi',
       ],
     }],
+    [ 'OS=="mac" and OS!="ios"', {
+      'includes': [
+        'mac/sandbox_mac.gypi',
+      ],
+    }],
     [ 'OS!="win" and OS!="mac" and OS!="linux" and OS!="android"', {
       # A 'default' to accomodate the "sandbox" target.
       'targets': [
         {
           'target_name': 'sandbox',
           'type': 'none',
         }
        ]
     }],
   ],
 }