Update PpdProvider threading model.

chromeos/printing/ppd_cache.cc

Index: chromeos/printing/ppd_cache.cc
diff --git a/chromeos/printing/ppd_cache.cc b/chromeos/printing/ppd_cache.cc
index 2b4abd1288111de8b4a9a3db9ea639f4b874e870..21d53dca806f7dc38c32855b332fc86da8b225f7 100644
--- a/chromeos/printing/ppd_cache.cc
+++ b/chromeos/printing/ppd_cache.cc
@@ -149,7 +149,8 @@ class PpdCacheImpl : public PpdCache {
     }
     if (base::WriteFile(available_printers_file_, contents.data(),
                         contents.size()) != static_cast<int>(contents.size())) {
-      LOG(ERROR) << "Failed to write available printers cache";
+      LOG(ERROR) << "Failed to write available printers cache to "
+                 << available_printers_file_.MaybeAsASCII();
     }
   }
 
@@ -219,7 +220,8 @@ class PpdCacheImpl : public PpdCache {
       if (!base::ReadFileToStringWithMaxSize(
               available_printers_file_, buf,
               options_.max_available_list_cached_size)) {
-        LOG(ERROR) << "Failed to read printer cache";
+        LOG(ERROR) << "Failed to read printer cache from "
+                   << available_printers_file_.MaybeAsASCII();
         buf->clear();
         return false;
       }