Convert PpdCache and PpdProvider to TaskScheduler.

chromeos/printing/ppd_provider.h

Index: chromeos/printing/ppd_provider.h
diff --git a/chromeos/printing/ppd_provider.h b/chromeos/printing/ppd_provider.h
index 26ae49fdc2f794264e473753fae97654526a8174..f7532889a9bfbb54d2bdf2763634439a24d920a8 100644
--- a/chromeos/printing/ppd_provider.h
+++ b/chromeos/printing/ppd_provider.h
@@ -12,7 +12,6 @@
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
-#include "base/sequenced_task_runner.h"
 #include "chromeos/chromeos_export.h"
 #include "chromeos/printing/printer_configuration.h"
 
@@ -91,7 +90,6 @@ class CHROMEOS_EXPORT PpdProvider : public base::RefCounted<PpdProvider> {
       const std::string& browser_locale,
       scoped_refptr<net::URLRequestContextGetter> url_context_getter,
       scoped_refptr<PpdCache> cache,
-      scoped_refptr<base::SequencedTaskRunner> disk_task_runner,
       const Options& options = Options());
 
   // Get all manufacturers for which we have drivers.  Keys of the map will be