Add PPDProvider barebones implementation and associated cache skeleton.

chromeos/printing/printer_configuration.h

Index: chromeos/printing/printer_configuration.h
diff --git a/chromeos/printing/printer_configuration.h b/chromeos/printing/printer_configuration.h
index bf4cbd250cbac8edfd20c41869e6e1e3257b1a2a..1d5bfcc727ea4d1bc5555aa711df9c190513e307 100644
--- a/chromeos/printing/printer_configuration.h
+++ b/chromeos/printing/printer_configuration.h
@@ -16,6 +16,18 @@ 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.