Set USE(NEW_THEME) in config.h instead of a gyp file

Source/wtf/wtf.gyp

 # Copyright (C) 2012 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 45 matching lines @@
             'defines': [
               '__STD_C',
               '_CRT_SECURE_NO_DEPRECATE',
               '_SCL_SECURE_NO_DEPRECATE',
               'CRASH=__debugbreak',
             ],
             'include_dirs': [
               'os-win32',
             ],
           }],
-          # FIXME: This should go in a header, not in the GYP file.
-          ['OS=="mac"', {
-            'defines': [
-              'WTF_USE_NEW_THEME=1',
-            ],
-          }],
         ],
       },
     },
     {
       'target_name': 'wtf',
       'type': '<(component)',
       'include_dirs': [
         '..',
       ],
       'dependencies': [
@@ skipping 75 matching lines @@
           'sources/': [
             ['exclude', 'Mac\\.mm$'],
             # mac is the only OS that uses WebKit's copy of TCMalloc.
             ['exclude', 'TC.*\\.(cpp|h)$'],
           ],
         }],
       ],
     },
   ]
 }