Add UMA stats for new Print Preview UI elements.

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 ed4d1e2f595be980157fb9e2ab94bbfd5de4cc25..8d3619994d554fcad3cdd27f9b2369fccd2762f8 100644
--- a/printing/print_job_constants.cc
+++ b/printing/print_job_constants.cc
@@ -95,6 +95,9 @@ const char kSettingMediaSizeWidthMicrons[] = "width_microns";
 // Key that specifies the requested media platform specific vendor id.
 const char kSettingMediaSizeVendorId[] = "vendor_id";
 
+// Key that specifies whether the requested media is a default one.
+const char kSettingMediaSizeIsDefault[] = "is_default";
+
 // Key that specifies the bottom margin of the page.
 const char kSettingMarginBottom[] = "marginBottom";