Update PpdProvider threading model.

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 ff6d6d3d0e88913efb11783c0969508de77eaecb..109364c9d216c4e9c59ba7b1fb5d50d4f5f32c10 100644
--- a/chromeos/printing/ppd_provider.h
+++ b/chromeos/printing/ppd_provider.h
@@ -12,6 +12,7 @@
 
 #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"
 
@@ -75,9 +76,12 @@ class CHROMEOS_EXPORT PpdProvider {
   };
 
   // Create and return a new PpdProvider with the given cache and options.
+  // |disk_task_runner| is used to run operations that hit the disk.  References
+  // to |url_context_getter| and |disk_task_runner| are taken.
   static std::unique_ptr<PpdProvider> Create(
       const std::string& api_key,
       scoped_refptr<net::URLRequestContextGetter> url_context_getter,
+      scoped_refptr<base::SequencedTaskRunner> disk_task_runner,

[stevenjb, 2016/11/11 19:26:44]

nit: file_task_runner is a more common term.

       std::unique_ptr<PpdCache> cache,
       const Options& options = Options());