Reinforce that ENABLE(OPENTYPE_VERTICAL) is a feature define

Source/build/features.gypi

 #
 # Copyright (C) 2009 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
@@ skipping 69 matching lines @@
         ],
       }],
       ['OS=="android" and use_openmax_dl_fft!=0', {
         'feature_defines': [
           'WTF_USE_WEBAUDIO_OPENMAX_DL_FFT=1',
           # Enabling the FFT is enough to enable WebAudio support to
           # allow most WebAudio features to work on Android.
           'ENABLE_WEB_AUDIO=1',
         ],
       }],
-      ['OS=="win" or OS=="android" or OS=="linux"', {
+      # Mac OS X has not implemented support for ENABLE(OPENTYPE_VERTICAL) yet
+      ['OS!="mac"', {
         'feature_defines': [
           'ENABLE_OPENTYPE_VERTICAL=1',
         ],
       }],
       ['use_default_render_theme==1', {
         'feature_defines': [
           'WTF_USE_DEFAULT_RENDER_THEME=1',
         ],
       }],
       ['enable_oilpan==1', {
         'feature_defines': [
           'ENABLE_OILPAN=1',
         ],
       }],
     ],
   },
 }