Keep pdf_engine in sync with PDF_ENABLE_XFA.

When the pdf_enable_xfa GYP_DEFINES flag is present, pass
-DPDF_ENABLE_XFA during the build of pdf engine. If the
XFA branch is present, the public XFA header will notice this
flag, and define PDF_USE_XFA in response, which the engine
tests to control its usage of XFA features.  When chrome is
built against master, PDF_USE_XFA is never defined, so this
still works regardless of the setting of the GYP define.

Review URL: https://codereview.chromium.org/1511443004

Cr-Commit-Position: refs/heads/master@{#364466}

pdf/BUILD.gn

@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//third_party/pdfium/pdfium.gni")
+
 pdf_engine = 0  # 0 PDFium
 
 static_library("pdf") {
@@ -46,6 +48,10 @@ static_library("pdf") {
     ]
   }
 
+  if (pdf_enable_xfa) {
+    defines = [ "PDF_ENABLE_XFA" ]
+  }
+
   deps = [
     "//base",
     "//components/ui/zoom",

pdf/pdf.gyp

 {
   'variables': {
     'chromium_code': 1,
+    'pdf_enable_xfa%': 0,
     'pdf_engine%': 0,  # 0 PDFium
   },
   'targets': [
@@ -55,6 +56,11 @@
             '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, ],