Add an autoconf field to printer objects.

chrome/browser/chromeos/printing/specifics_translation.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_SPECIFICS_TRANSLATION_H_
 #define CHROME_BROWSER_CHROMEOS_PRINTING_SPECIFICS_TRANSLATION_H_
 
 #include <memory>
 
 #include "chromeos/printing/printer_configuration.h"
 #include "components/sync/protocol/printer_specifics.pb.h"
 
 namespace chromeos {
 namespace printing {
 
+// Convert |printer| into its local representation.  Enforces that only one
+// field in PpdReference is filled in.  In order of preference, we populate
+// autoconf, user_supplied_ppd_url, or effective_make_and_model.
 std::unique_ptr<Printer> SpecificsToPrinter(
     const sync_pb::PrinterSpecifics& printer);
 
+// Convert |printer| into its proto representation.
 std::unique_ptr<sync_pb::PrinterSpecifics> PrinterToSpecifics(
     const Printer& printer);
 
-// Merge fields from |printer| into |specifics|.
+// Merge fields from |printer| into |specifics|.  Merge strategy is to only
+// write non-default fields from |printer| into the appropriate field in
+// |specifics|.  Default fields are skipped to prevent accidentally clearing
+// |specifics|.  Enforces field exclusivity in PpdReference as described in
+// SpecificsToPrinter.
 void MergePrinterToSpecifics(const Printer& printer,
                              sync_pb::PrinterSpecifics* specifics);
 
 }  // namespace printing
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_PRINTING_SPECIFICS_TRANSLATION_H_