Keep pdf_engine in sync with PDF_ENABLE_XFA.

pdf/BUILD.gn

Index: pdf/BUILD.gn
diff --git a/pdf/BUILD.gn b/pdf/BUILD.gn
index 593e6ba570ebca37f98dd17a459732754ae90e9b..194803a2079153f37884ec2ed1c879751c85b7dd 100644
--- a/pdf/BUILD.gn
+++ b/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",