Keep pdf_engine in sync with PDF_ENABLE_XFA.

pdf/pdf.gyp

 {
   'variables': {
     'chromium_code': 1,
+    'pdf_enable_xfa%': 0,
     'pdf_engine%': 0,  # 0 PDFium
   },
   'targets': [
     {
       'target_name': 'pdf',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         '../components/components.gyp:ui_zoom',
         '../content/content.gyp:content_common',
@@ skipping 34 matching lines @@
             'pdfium/pdfium_mem_buffer_file_read.cc',
             'pdfium/pdfium_mem_buffer_file_read.h',
             'pdfium/pdfium_mem_buffer_file_write.cc',
             'pdfium/pdfium_mem_buffer_file_write.h',
             'pdfium/pdfium_page.cc',
             'pdfium/pdfium_page.h',
             'pdfium/pdfium_range.cc',
             'pdfium/pdfium_range.h',
           ],
         }],
+        ['pdf_enable_xfa==1', {
+           'defines': [
+             'PDF_ENABLE_XFA',
+           ]
+        }],
         ['OS=="win"', {
           # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
           'msvs_disabled_warnings': [ 4267, ],
         }],
       ],
     },
   ],
 }