Convert sbox_unittests to run exclusively on Swarming

sandbox/win/sandbox_win.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.
 
 {
   'target_defaults': {
     'variables': {
       'sandbox_windows_target': 0,
       'target_arch%': 'ia32',
     },
@@ skipping 364 matching lines @@
           'dependencies': [
             'sbox_integration_tests',
           ],
           'includes': [
             '../../build/isolate.gypi',
           ],
           'sources': [
             '../sbox_integration_tests.isolate',
           ],
         },
+        {
+          'target_name': 'sbox_unittests_run',
+          'type': 'none',
+          'dependencies': [
+            'sbox_unittests',
+          ],
+          'includes': [
+            '../../build/isolate.gypi',
+          ],
+          'sources': [
+            '../sbox_unittests.isolate',
+          ],
+        },
       ],
     }],
   ],
 }