Add PPDProvider barebones implementation and associated cache skeleton.

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>
 
 #include "base/macros.h"
 #include "chromeos/chromeos_export.h"
 
 namespace chromeos {
 
 class CHROMEOS_EXPORT Printer {
  public:
   // Information needed to find the PPD file for this printer.
+  //
+  // If you add fields to this struct, you almost certainly will
+  // want to update PpdResolver and PpdCache::GetCachePath.
+  //
+  // At resolution time, we look for a cached PPD that used the same
+  // PpdReference before.
+  //
+  // If one is not found and user_supplied_ppd_url is set, we'll fail
+  // out with NOT FOUND
+  //
+  // Otherwise, we'll hit QuirksServer to see if we can resolve a Ppd
+  // using manufacturer/model
   struct PpdReference {
     // If non-empty, this is the url of a specific PPD the user has specified
     // for use with this printer.
     std::string user_supplied_ppd_url;
 
     // The *effective* manufacturer and model of this printer, in other words,
     // the manufacturer and model of the printer for which we grab a PPD.  This
     // doesn’t have to (but can) be the actual manufacturer/model of the
     // printer.  We should always try to fill these fields in, even if we don’t
     // think they’ll be needed, as they provide a fallback mechanism for
@@ skipping 67 matching lines @@
   // How to find the associated postscript printer description.
   PpdReference ppd_reference_;
 
   // The UUID from an autoconf protocol for deduplication. Could be empty.
   std::string uuid_;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_PRINTING_PRINTER_CONFIGURATION_H_