Make the dependency of enable_print_preview and enable_pdf explicit

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 0ca8e18a5bb1ecef7d74ecac665bcab4a6d07a69..0b3b86df03a6479febe8b6e26634d7d0a9c87912 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -69,6 +69,8 @@ config("feature_flags") {
   }
   if (enable_pdf) {
     defines += [ "ENABLE_PDF=1" ]
+  } else {
+    assert(!is_win, "set enable_pdf=true on Windows.")

[Lei Zhang, 2016/05/18 21:43:37]

I'd prefer we remove this check, and put all the checks below in the printing
block. Essentially:

- Windows basic printing requires PDF
- Print preview requires PDF

[Wei Li, 2016/05/19 17:13:30]

Done.

   }
   if (enable_basic_printing || enable_print_preview) {
     # Convenience define for ENABLE_BASIC_PRINTING || ENABLE_PRINT_PREVIEW.
@@ -81,6 +83,8 @@ config("feature_flags") {
       # Enable printing with print preview.
       # Can be defined without ENABLE_BASIC_PRINTING.
       defines += [ "ENABLE_PRINT_PREVIEW=1" ]
+      assert(enable_pdf,
+             "Print preview depends on pdf module: set enable_pdf=true.")
     }
   }
   if (enable_spellcheck) {