PrintPreview: Added code to identify the printer default duplex value.

chrome/browser/printing/print_dialog_gtk.cc

Index: chrome/browser/printing/print_dialog_gtk.cc
diff --git a/chrome/browser/printing/print_dialog_gtk.cc b/chrome/browser/printing/print_dialog_gtk.cc
index 4c3bb828199fc00e5a1fcc8616b5fcd6c3ddadd2..27b3dbe16381c3343e02e107937e9e07ea88b66d 100644
--- a/chrome/browser/printing/print_dialog_gtk.cc
+++ b/chrome/browser/printing/print_dialog_gtk.cc
@@ -208,11 +208,14 @@ bool PrintDialogGtk::UpdateSettings(const DictionaryValue& settings,
       case printing::SHORT_EDGE:
         cups_duplex_mode = kDuplexTumble;
         break;
-      default:
+      case printing::SIMPLEX:
         cups_duplex_mode = kDuplexNone;
         break;
+      default:  // UNKNOWN_DUPLEX_MODE
+        break;
     }
-    gtk_print_settings_set(gtk_settings_, kCUPSDuplex, cups_duplex_mode);
+    if (duplex_mode != printing::UNKNOWN_DUPLEX_MODE)
+      gtk_print_settings_set(gtk_settings_, kCUPSDuplex, cups_duplex_mode);
   }
 
   gtk_print_settings_set_orientation(