Adds label_type: Group to gn_isolate_map.pyl

testing/buildbot/gn_isolate_map.pyl

 ## Copyright 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.
 
 # This is a .pyl, or "Python Literal", file. You can treat it just like a
 # .json file, with the following exceptions:
 # * all keys must be quoted (use single quotes, please);
 # * comments are allowed, using '#' syntax; and
 # * trailing commas are allowed.
 
@@ skipping 132 matching lines @@
   "ash_content_unittests": {
     "label": "//ash:ash_content_unittests",
     "type": "windowed_test_launcher",
   },
   "ash_unittests": {
     "label": "//ash:ash_unittests",
     "type": "windowed_test_launcher",
   },
   "ash_mus_unittests": {
     "label": "//ash:ash_unittests",
+    "label_type": "group",
     "type": "windowed_test_launcher",
     "args": [
       "--mus",
     ],
     "executable": "ash_unittests",
   },
   "audio_unittests": {
     "label": "//media:audio_unittests",
     "type": "raw",
     "args": [],
@@ skipping 942 matching lines @@
   },
   "wm_unittests": {
     "label": "//ui/wm:wm_unittests",
     "type": "windowed_test_launcher",
   },
   "wtf_unittests": {
     "label": "//third_party/WebKit/Source/platform/wtf:wtf_unittests",
     "type": "console_test_launcher",
   },
 }