Add PPDProvider barebones implementation and associated cache skeleton.

chromeos/printing/printer_translator_unittest.cc

Index: chromeos/printing/printer_translator_unittest.cc
diff --git a/chromeos/printing/printer_translator_unittest.cc b/chromeos/printing/printer_translator_unittest.cc
index 51edff9df7d5716b97080cb05b937dac2af39ed1..4b5556296af7322c084f47676579de05e6324cf4 100644
--- a/chromeos/printing/printer_translator_unittest.cc
+++ b/chromeos/printing/printer_translator_unittest.cc
@@ -104,8 +104,6 @@ TEST(PrinterTranslatorTest, PrinterToPrefPpdReferenceLazy) {
   EXPECT_FALSE(actual->HasKey("ppd_reference.effective_model"));
 }
 
-// FIXME add a test that we don't set fields that are empty.
-
 TEST(PrinterTranslatorTest, PrefToPrinterRoundTrip) {
   base::DictionaryValue preference;
   preference.SetString("id", kGUID);