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..85f405a623841be2d6fe23a1c0192194d8c7de34 100644
--- a/chrome/browser/chromeos/printing/printer_discoverer.cc
+++ b/chrome/browser/chromeos/printing/printer_discoverer.cc
@@ -59,7 +59,8 @@ class PrinterDiscovererImpl : public PrinterDiscoverer,
     usb_printers_ = printers;
     std::vector<Printer> all_printers = GetAvailablePrinters();
     for (PrinterDiscoverer::Observer& observer : observer_list_) {
-      observer.OnPrintersFound(all_printers);
+      // We get all USB printers the first time so the scan is completed.
+      observer.OnPrintersFound(true /*scan_done*/, all_printers);
     }
   }
 
@@ -69,13 +70,12 @@ class PrinterDiscovererImpl : public PrinterDiscoverer,
   void WrappedOnPrintersFound(PrinterDiscoverer::Observer* observer,
                               const std::vector<Printer>& printers) {
     if (observer_list_.HasObserver(observer)) {
-      observer->OnPrintersFound(printers);
       // Since USB is the only thing we're worried about at the moment, and we
       // don't have to wait for those printers to be scanned, we can just tell
       // 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->OnPrintersFound(true, printers);
     }
   }