Move enable_pdf to a buildflag header.

printing/features/features.gni

Index: printing/features/features.gni
diff --git a/printing/features/features.gni b/printing/features/features.gni
index 651b7a7a221169526a914fd03227e8cadc8e0559..a41202a2e194451f33942a76e93adef67037d521 100644
--- a/printing/features/features.gni
+++ b/printing/features/features.gni
@@ -15,10 +15,3 @@ declare_args() {
 
   use_cups = (is_desktop_linux || is_mac) && !is_chromecast
 }
-
-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.")
-}