Add support for starting an extension print job to print preview

printing/print_job_constants.cc

Index: printing/print_job_constants.cc
diff --git a/printing/print_job_constants.cc b/printing/print_job_constants.cc
index 3caadd10d38677d358a979378eb0dcfa2c9556b1..ffaa88513d17f9699f50282b267940c4e2977234 100644
--- a/printing/print_job_constants.cc
+++ b/printing/print_job_constants.cc
@@ -160,6 +160,10 @@ const char kSettingPrintToPDF[] = "printToPDF";
 // not.
 const char kSettingPrintWithPrivet[] = "printWithPrivet";
 
+// Print using extension option: true if destination is an extension printer,
+// false if not.
+const char kSettingPrintWithExtension[] = "printWithExtension";
+
 // Ticket option. Contains the ticket in CJT format.
 const char kSettingTicket[] = "ticket";