Printing: Add ability to get the default printer from the print backend.

printing/backend/print_backend_cups.cc

Index: printing/backend/print_backend_cups.cc
===================================================================
--- printing/backend/print_backend_cups.cc	(revision 87114)
+++ printing/backend/print_backend_cups.cc	(working copy)
@@ -86,6 +86,8 @@
   // PrintBackend implementation.
   virtual bool EnumeratePrinters(PrinterList* printer_list);
 
+  virtual std::string GetDefaultPrinterName();
+
   virtual bool GetPrinterCapsAndDefaults(const std::string& printer_name,
                                          PrinterCapsAndDefaults* printer_info);
 
@@ -153,6 +155,13 @@
   return true;
 }
 
+std::string PrintBackendCUPS::GetDefaultPrinterName() {
+  // TODO(thestig) Figure out why cupsGetDefault() lies about the default
+  // printer. :-(
+  // Return an empty string for now.
+  return std::string();
+}
+
 bool PrintBackendCUPS::GetPrinterCapsAndDefaults(
     const std::string& printer_name,
     PrinterCapsAndDefaults* printer_info) {