Use worker pool for IO in JsonPrefStore.

chrome/service/service_process_prefs.h

Index: chrome/service/service_process_prefs.h
diff --git a/chrome/service/service_process_prefs.h b/chrome/service/service_process_prefs.h
index e5aecde5d7d35b069cddf985d190b1ae11de07c9..7bc4c4b6545c0cfc555eee8f2daf9d1afb86b513 100644
--- a/chrome/service/service_process_prefs.h
+++ b/chrome/service/service_process_prefs.h
@@ -12,7 +12,7 @@
 
 namespace base {
 class DictionaryValue;
-}
+}  // namespace base
 
 // Manages persistent preferences for the service process. This is basically a
 // thin wrapper around JsonPrefStore for more comfortable use.
@@ -21,7 +21,7 @@ class ServiceProcessPrefs {
   // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
   // file I/O can be done.
   ServiceProcessPrefs(const FilePath& pref_filename,
-                      base::MessageLoopProxy* file_message_loop_proxy);
+                      base::SequencedTaskRunner* blocking_task_runner);
   ~ServiceProcessPrefs();
 
   // Read preferences from the backing file.