Move default value of pdf_enable_xfa to standalone.gypi

pdfium.gyp

 # Copyright 2015 PDFium 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': {
     # TODO(thakis): Enable this, pdfium:29
     #'chromium_code': 1,
     'pdf_use_skia%': 0,
     'pdf_enable_v8%': 1,
-    'pdf_enable_xfa%': 1,
+    'pdf_enable_xfa%': 0, # Set to 1 by standalone.gypi in a standalone build.
     'conditions': [
       ['OS=="linux"', {
         'bundle_freetype%': 0,
       }, {  # On Android there's no system FreeType. On Windows and Mac, only a
             # few methods are used from it.
         'bundle_freetype%': 1,
       }],
     ],
   },
   'target_defaults': {
@@ skipping 847 matching lines @@
           ],
           'dependencies': [
             '<(DEPTH)/v8/tools/gyp/v8.gyp:v8',
             '<(DEPTH)/v8/tools/gyp/v8.gyp:v8_libplatform',
           ],
         }],
       ],
     },
   ],
 }