Add Printing.CUPS.PrintersDiscovered to our metrics.

chrome/browser/chromeos/printing/printer_discoverer.cc

Index: chrome/browser/chromeos/printing/printer_discoverer.cc
diff --git a/chrome/browser/chromeos/printing/printer_discoverer.cc b/chrome/browser/chromeos/printing/printer_discoverer.cc
index e6f91591cd6080de08144e1cabeaae076bc3bf26..67812498b57a95a72ed428cdac5a6c287052b8ee 100644
--- a/chrome/browser/chromeos/printing/printer_discoverer.cc
+++ b/chrome/browser/chromeos/printing/printer_discoverer.cc
@@ -75,7 +75,7 @@ class PrinterDiscovererImpl : public PrinterDiscoverer,
       // the observer the initial scan is done now.  This will change when we're
       // also doing network discovery -- we'll hold off on issuing this callback
       // until the network discovery is done as well.
-      observer->OnDiscoveryInitialScanDone();
+      observer->OnDiscoveryInitialScanDone(printers.size());
     }
   }