When printing a PDF, query and use the print scaling disabled option.

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 f07a4608477381d8a463197cf626eb49e87bcca9..adebd1406c53b12a92251dae25f1078e2c603863 100644
--- a/printing/print_job_constants.cc
+++ b/printing/print_job_constants.cc
@@ -111,6 +111,8 @@ const char kSettingPageRangeFrom[] = "from";
 // The last page of a page range. (1-based)
 const char kSettingPageRangeTo[] = "to";
 
+const char kSettingPreviewModifiable[] = "previewModifiable";
+
 // Printer name.
 const char kSettingPrinterName[] = "printerName";