Add metrics to printer setup flow.

chromeos/printing/printer_configuration.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 CHROMEOS_PRINTING_PRINTER_CONFIGURATION_H_
 #define CHROMEOS_PRINTING_PRINTER_CONFIGURATION_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 29 matching lines @@
     // True if the printer should be auto-configured and a PPD is unnecessary.
     bool autoconf = false;
   };
 
   // The location where the printer is stored.
   enum Source {
     SRC_USER_PREFS,
     SRC_POLICY,
   };
 
-  // An enumeration of printer protocols.  Do not change these values as they
-  // are used in enums.xml.
+  // An enumeration of printer protocols.
+  // These values are written to logs.  New enum values can be added, but
+  // existing enums must never be renumbered or deleted and reused.
   enum PrinterProtocol {
     kUnknown = 0,
     kUsb = 1,
     kIpp = 2,
     kIpps = 3,
     kHttp = 4,
     kHttps = 5,
     kSocket = 6,
     kLpd = 7,
     kProtocolMax
@@ skipping 85 matching lines @@
   // The datastore which holds this printer.
   Source source_;
 
   // Timestamp of most recent change.
   base::Time last_updated_;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_PRINTING_PRINTER_CONFIGURATION_H_