Add Printing.CUPS.PrintersDiscovered to our metrics.

chrome/browser/chromeos/printing/printer_discoverer.h

Index: chrome/browser/chromeos/printing/printer_discoverer.h
diff --git a/chrome/browser/chromeos/printing/printer_discoverer.h b/chrome/browser/chromeos/printing/printer_discoverer.h
index 20fd8bc9b5bf9096e3489c249a4e506dad41f9df..c2b13138bf602de61322ccbf08e04e95fab6f1aa 100644
--- a/chrome/browser/chromeos/printing/printer_discoverer.h
+++ b/chrome/browser/chromeos/printing/printer_discoverer.h
@@ -24,20 +24,18 @@ class CHROMEOS_EXPORT PrinterDiscoverer {
    public:
     virtual ~Observer() = default;
 
-    // Called when we are done with the initial scan for printers.  We may
-    // still call OnPrintersFound if the set of available printers
-    // changes, but the user can conclude that if a printer is currently
-    // available and not in the list, we're not still looking for it.
-    virtual void OnDiscoveryInitialScanDone() = 0;
-
     // Called with a collection of printers as they are discovered.  On each
     // call |printers| is the full set of known printers; it is not
-    // incremental; printers may be added or removed.
+    // incremental; printers may be added or removed.  |scan_done| is true after
+    // the initial scan has been completed.  Observers will not receive
+    // notifications after scan completion unless the number of printers
+    // detected by the discoverer changes.
     //
     // Observers will get an OnPrintersFound callback after registration
     // with the existing list of printers (which may be empty) and will get
     // additional calls whenever the set of printers changes.
-    virtual void OnPrintersFound(const std::vector<Printer>& printers) = 0;
+    virtual void OnPrintersFound(bool scan_done,
+                                 const std::vector<Printer>& printers) = 0;
   };
 
   // Static factory