Revert 135222 - it broke multiprofile tests - Use worker pool for IO in JsonPrefStore.

chrome/service/service_process_prefs.h

Index: chrome/service/service_process_prefs.h
===================================================================
--- chrome/service/service_process_prefs.h	(revision 135722)
+++ chrome/service/service_process_prefs.h	(working copy)
@@ -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 @@
   // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
   // file I/O can be done.
   ServiceProcessPrefs(const FilePath& pref_filename,
-                      base::SequencedTaskRunner* blocking_task_runner);
+                      base::MessageLoopProxy* file_message_loop_proxy);
   ~ServiceProcessPrefs();
 
   // Read preferences from the backing file.