Enable the cups backend when --enable-native-cups is specified.

printing/backend/print_backend.h

Index: printing/backend/print_backend.h
diff --git a/printing/backend/print_backend.h b/printing/backend/print_backend.h
index 5086d2047346f111cdd7ba5ed8bb94a0c5c2a9a1..46b0801323db12cec7751f5108d54adb99f00100 100644
--- a/printing/backend/print_backend.h
+++ b/printing/backend/print_backend.h
@@ -123,6 +123,11 @@ class PRINTING_EXPORT PrintBackend
   static scoped_refptr<PrintBackend> CreateInstance(
       const base::DictionaryValue* print_backend_settings);
 
+  // Returns the value of the native cups flag
+  static bool GetNativeCupsEnabled();
+
+  static void SetNativeCupsEnabled(bool enabled);
+
  protected:
   friend class base::RefCountedThreadSafe<PrintBackend>;
   virtual ~PrintBackend();