Move enable_pdf to a buildflag header.

printing/BUILD.gn

Index: printing/BUILD.gn
diff --git a/printing/BUILD.gn b/printing/BUILD.gn
index b3ffe9529b2ef62f362efb02d5c9c3c6280a29b7..159f4364bbde8aa4fc46c125e583acc77ef33ebb 100644
--- a/printing/BUILD.gn
+++ b/printing/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/config/features.gni")
 import("//build/config/sysroot.gni")
 import("//build/config/ui.gni")
+import("//pdf/features.gni")
 import("//printing/features/features.gni")
 import("//testing/test.gni")
 if (is_mac) {
@@ -14,6 +15,13 @@ if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
+if ((enable_basic_printing && is_win) || enable_print_preview) {
+  # Windows basic printing or print preview requires pdf enabled.
+  assert(enable_pdf,
+         "Windows basic printing or print preview needs pdf: " +
+             "set enable_pdf=true.")
+}
+
 component("printing") {
   sources = [
     "backend/print_backend.cc",