Start building message_center 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 125 matching lines @@
             'enable_touch_ui%': 1,
           }],
 
           # Enable App Launcher only on ChromeOS, Windows and OSX.
           ['use_ash==1 or OS=="win" or OS=="mac"', {
             'enable_app_list%': 1,
           }, {
             'enable_app_list%': 0,
           }],
 
-          # Enable Message Center only on ChromeOS and Windows for now.
-          ['use_ash==1 or OS=="win"', {
+          # Enable Message Center only on ChromeOS, Windows, and Mac for now.
+          ['use_ash==1 or OS=="win" or OS=="mac"', {
             'enable_message_center%': 1,
           }, {
             'enable_message_center%': 0,
           }],
 
           ['use_aura==1 or (OS!="win" and OS!="mac" and OS!="ios" and OS!="android")', {
             'use_default_render_theme%': 1,
           }, {
             'use_default_render_theme%': 0,
           }],
@@ skipping 3999 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',
   },
 }