Add pdf layer code for PDFium XFA module

pdf/BUILD.gn

 # Copyright 2014 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.
 
 pdf_engine = 0  # 0 PDFium
+pdf_use_xfa = 0
 
 # TODO(GYP) need support for loadable modules
 shared_library("pdf") {
   sources = [
     "button.h",
     "button.cc",
     "chunk_stream.h",
     "chunk_stream.cc",
     "control.h",
     "control.cc",
@@ skipping 47 matching lines @@
       "pdfium/pdfium_range.cc",
       "pdfium/pdfium_range.h",
     ]
   }
 
   if (is_win) {
     defines = [ "COMPILE_CONTENT_STATICALLY" ]
     cflags = [ "/wd4267" ]  # TODO(jschuh) size_t to int truncations.
   }
 
+  if (pdf_use_xfa == 1) {
+    defines += [ "PDF_USE_XFA" ]
+  }
+
   if (is_mac) {
     # TODO(GYP)
     #'mac_bundle': 1,
     #'product_name': 'PDF',
     #'product_extension': 'plugin',
     ## Strip the shipping binary of symbols so "Foxit" doesn't appear in
     ## the binary.  Symbols are stored in a separate .dSYM.
     #'variables': {
     #  'mac_real_dsym': 1,
     #},
     #'sources+': [
     #  'Info.plist'
     #]
     #'xcode_settings': {
     #  'INFOPLIST_FILE': 'Info.plist',
     #},
   }
 
   deps = [
     "//base",
     "//net",
     "//ppapi:ppapi_cpp",
     "//third_party/pdfium",
   ]
 }
 
 # TODO(GYP) pdf_linux_symbols target.