use toolkit_uses_gtk==1 instead of OS==linux for printing_context_gtk.{cc,h}

printing/printing.gyp

 # Copyright (c) 2011 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
@@ skipping 160 matching lines @@
         ['OS=="linux" and chromeos==1', {
           'defines': [
             # PRINT_BACKEND_AVAILABLE disables the default dummy implementation
             # of the print backend and enables a custom implementation instead.
             'PRINT_BACKEND_AVAILABLE',
           ],
           'sources': [
             'backend/print_backend_chromeos.cc',
           ],
         }],
-        ['OS=="linux" and chromeos==0', {
+        ['toolkit_uses_gtk==1 and chromeos==0', {
           'sources': [
             'printing_context_gtk.cc',
             'printing_context_gtk.h',
           ],
         }],
       ],
     },
     {
       'target_name': 'printing_unittests',
       'type': 'executable',
@@ skipping 65 matching lines @@
                   '<!@(python cups_config_helper.py --cflags)',
                 ],
               }],
             ],
           },
         }],
       ],
     },
   ],
 }