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..613733ac6bb9b2828cfb7d15acd81b51687ed620 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.

[skau, 2016/11/09 00:01:09]

If you're taking a reference to these objects, you should pass the scoped_refptr
https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md

[Carlson, 2016/11/10 19:22:50]

Ugh, I had found (apparently older) advice that said just the opposite.  Changed
back.

   static std::unique_ptr<PpdProvider> Create(
       const std::string& api_key,
-      scoped_refptr<net::URLRequestContextGetter> url_context_getter,
+      net::URLRequestContextGetter* url_context_getter,
+      base::SequencedTaskRunner* disk_task_runner,
       std::unique_ptr<PpdCache> cache,
       const Options& options = Options());