Update PpdProvider threading model.

chromeos/printing/ppd_provider.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROMEOS_PRINTING_PPD_PROVIDER_H_
 #define CHROMEOS_PRINTING_PPD_PROVIDER_H_
 
 #include <map>
 #include <memory>
 #include <string>
 #include <vector>
 
 #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"
 
 namespace net {
 class URLRequestContextGetter;
 }
 
 namespace chromeos {
 namespace printing {
 
@@ skipping 43 matching lines @@
 
     // hostname of quirks server to query.
     std::string quirks_server = "chromeosquirksserver-pa.googleapis.com";
 
     // Maximum size of the contents of a PPD file, in bytes.  Trying to use a
     // PPD file bigger than this will cause INTERNAL_ERRORs at resolution time.
     size_t max_ppd_contents_size_ = 100 * 1024;
   };
 
   // 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());
 
   virtual ~PpdProvider() {}
 
   // Given a PpdReference, attempt to resolve the PPD for printing.
   //
   // Must be called from a Sequenced Task context (i.e.
   // base::SequencedTaskRunnerHandle::IsSet() must be true).
   //
@@ skipping 31 matching lines @@
   // whatever they give us directly into the cache without doing a resolve.
   // This hook lets is do that.
   virtual bool CachePpd(const Printer::PpdReference& ppd_reference,
                         const base::FilePath& ppd_path) = 0;
 };
 
 }  // namespace printing
 }  // namespace chromeos
 
 #endif  // CHROMEOS_PRINTING_PPD_PROVIDER_H_