Add metrics to printer setup flow.

chrome/browser/chromeos/printing/printer_configurer.h

 // Copyright 2017 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_CONFIGURER_H_
 #define CHROME_BROWSER_CHROMEOS_PRINTING_PRINTER_CONFIGURER_H_
 
 #include <memory>
 
 #include "base/callback_forward.h"
 #include "chromeos/printing/printer_configuration.h"
 
 class Profile;
 
 namespace chromeos {
 
+// These values are written to logs.  New enum values can be added, but existing
+// enums must never be renumbered or deleted and reused.
 enum PrinterSetupResult {
   kFatalError = 0,          // Setup failed in an unrecognized way
   kSuccess = 1,             // Printer set up successfully
   kPrinterUnreachable = 2,  // Could not reach printer
   kDbusError = 3,           // Could not contact debugd
   // Space left for additional errors
 
   // PPD errors
   kPpdTooLarge = 10,       // PPD exceeds size limit
   kInvalidPpd = 11,        // PPD rejected by cupstestppd
   kPpdNotFound = 12,       // Could not find PPD
   kPpdUnretrievable = 13,  // Could not download PPD
   kMaxValue = kPpdUnretrievable

[Carlson, 2017/05/26 16:58:38]

I"m suddenly a little suspicious that kMaxValue here is set to the last value,
whereas in the other enums it's set to last+1.  Is the difference intentional?

[skau, 2017/05/26 17:18:02]

Well, this is the comment on the histogram method: The value in |sample| must be
strictly less than |enum_max|.

Annoyingly, if I write a switch for PrinterSetupResult, I need to handle
kMaxValue as an unexpected value.  However, the histogram macros expect last +
1.

 };
 
 using PrinterSetupCallback = base::Callback<void(PrinterSetupResult)>;
 
 // Configures printers by retrieving PPDs and registering the printer with CUPS.
 // Class must be constructed and used on the UI thread.
 class PrinterConfigurer {
  public:
   static std::unique_ptr<PrinterConfigurer> Create(Profile* profile);
 
   PrinterConfigurer(const PrinterConfigurer&) = delete;
   PrinterConfigurer& operator=(const PrinterConfigurer&) = delete;
 
   virtual ~PrinterConfigurer() = default;
 
   // Set up |printer| retrieving the appropriate PPD and registering the printer
   // with CUPS.  |callback| is called with the result of the operation.  This
   // method must be called on the UI thread and will run |callback| on the
   // UI thread.
   virtual void SetUpPrinter(const Printer& printer,
                             const PrinterSetupCallback& callback) = 0;
 
  protected:
   PrinterConfigurer() = default;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_PRINTING_PRINTER_CONFIGURER_H_