Let test_isolation_mode default to check on mac.

build/common.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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 738 matching lines @@
         }],
 
         # Whether tests targets should be run, archived or just have the
         # dependencies verified. All the tests targets have the '_run' suffix,
         # e.g. base_unittests_run runs the target base_unittests. The test
         # target always calls tools/swarming_client/isolate.py. See the script's
         # --help for more information and the valid --mode values. Meant to be
         # overriden with GYP_DEFINES.
         # TODO(maruel): Remove the conditions as more configurations are
         # supported.
-        # TODO(csharp): Remove OS!="mac" once xcode can run the isolate code
-        # again.
         # NOTE: The check for disable_nacl==0 and component=="static_library"
         # can't be used here because these variables are not defined yet, but it
         # is still not supported.
-        ['OS!="mac" and OS!="ios" and OS!="android" and chromeos==0', {
+        ['OS!="ios" and OS!="android" and chromeos==0', {
           'test_isolation_mode%': 'check',
         }, {
           'test_isolation_mode%': 'noop',
         }],
         # Whether Android ARM build uses OpenMAX DL FFT.
         ['OS=="android" and target_arch=="arm" and android_webview_build==0', {
           # Currently only supported on Android ARM, without webview.
           # When enabled, this will also enable WebAudio on Android
           # ARM.  Default is enabled.
           'use_openmax_dl_fft%': 1,
@@ skipping 3970 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }