Add Printing.CUPS.PrintersDiscovered to our metrics.

chrome/browser/chromeos/printing/printer_discoverer.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_CHROMEOS_PRINTING_PRINTER_DISCOVERER_H_
 #define CHROME_BROWSER_CHROMEOS_PRINTING_PRINTER_DISCOVERER_H_
 
 #include <memory>
 #include <vector>
 
 #include "chromeos/printing/printer_configuration.h"
 
 class Profile;
 
 namespace chromeos {
 
 // Interface for printer discovery.  Constructs Printer objects from USB and
 // zeroconf (DNS-SD) printers.  All functions in this class must be called
 // from a sequenced context.
 class CHROMEOS_EXPORT PrinterDiscoverer {
  public:
   // Interface for objects interested in detected printers.
   class Observer {
    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
   static std::unique_ptr<PrinterDiscoverer> CreateForProfile(Profile* profile);
 
   virtual ~PrinterDiscoverer() = default;
 
   // Add an observer that will be notified of discovered printers.  Ownership of
   // |observer| is not taken by the discoverer.  It is an error to add an
   // observer more than once.  Calls to |observer| methods will take place on
   // the thread PrinterDiscoverer was instantiated on.
   virtual void AddObserver(PrinterDiscoverer::Observer* observer) = 0;
 
   // Remove an observer of printer discovery.
   virtual void RemoveObserver(PrinterDiscoverer::Observer* observer) = 0;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_PRINTING_PRINTER_DISCOVERER_H_