add SK_SUPPORT_LEGACY_PROCXFERMODE to skia_for_chromium_defines.gypi

gyp/skia_for_chromium_defines.gypi

 # Copyright 2014 Google Inc.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This file is included by chrome's skia/skia_common.gypi, and is intended to
 # augment the skia flags that are set there.
 
 {
   'variables': {
 
     # These flags will be defined in chromium
     #
     # If these become 'permanent', they should be moved into skia_common.gypi
     #
     'skia_for_chromium_defines': [
       'SK_SUPPORT_LEGACY_PUBLICEFFECTCONSTRUCTORS=1',
       'SK_SUPPORT_LEGACY_GETTOPDEVICE',
       'SK_SUPPORT_LEGACY_PICTURE_CAN_RECORD',
       'SK_SUPPORT_DEPRECATED_RECORD_FLAGS',
       'SK_SUPPORT_LEGACY_N32_NAME',
+      'SK_SUPPORT_LEGACY_PROCXFERMODE',
     ],
   },
 }