Provide pdf_enable_xfa GN variable on master, too.

That way, pdf/BUILD.gn can test it no matter which branch
of PDFium is pulled under a chrome checkout.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1511183004 .
diff --git a/pdfium.gni b/pdfium.gni
index d70131d..ec71db9 100644
--- a/pdfium.gni
+++ b/pdfium.gni
@@ -12,6 +12,9 @@
   # Build PDFium either with or without v8 support.
   pdf_enable_v8 = true
 
+  # Build PDFium either with or without XFA Forms support.
+  pdf_enable_xfa = false
+
   # Build PDFium against skia (experimental) rather than agg.
   pdf_use_skia = false
 }