Convert sandbox_mac_unittests to run exclusively on Swarming

sandbox/sandbox_mac_unittests.isolate

Index: sandbox/sandbox_mac_unittests.isolate
diff --git a/sandbox/sandbox_mac_unittests.isolate b/sandbox/sandbox_mac_unittests.isolate
new file mode 100644
index 0000000000000000000000000000000000000000..dd1130bad6d853324d60a547732f4269d167756d
--- /dev/null
+++ b/sandbox/sandbox_mac_unittests.isolate
@@ -0,0 +1,10 @@
+# Copyright (c) 2015 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': {
+    'command': [ '<(PRODUCT_DIR)/sandbox_mac_unittests' ],
+    'files': [ '<(PRODUCT_DIR)/sandbox_mac_unittests' ],
+  },
+  'includes': [ '../base/base.isolate' ],
+}