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

chrome/browser/prefs/pref_service.cc

Index: chrome/browser/prefs/pref_service.cc
===================================================================
--- chrome/browser/prefs/pref_service.cc	(revision 135722)
+++ chrome/browser/prefs/pref_service.cc	(working copy)
@@ -17,7 +17,6 @@
 #include "base/stl_util.h"
 #include "base/string_number_conversions.h"
 #include "base/string_util.h"
-#include "base/threading/sequenced_worker_pool.h"
 #include "base/value_conversions.h"
 #include "build/build_config.h"
 #include "chrome/browser/browser_process.h"
@@ -146,10 +145,9 @@
 
   CommandLinePrefStore* command_line =
       new CommandLinePrefStore(CommandLine::ForCurrentProcess());
-  scoped_refptr<base::SequencedTaskRunner> io_task_runner =
-      BrowserThread::GetBlockingPool()->GetSequencedTaskRunner(
-          BrowserThread::GetBlockingPool()->GetSequenceToken());
-  JsonPrefStore* user = new JsonPrefStore(pref_filename, io_task_runner);
+  JsonPrefStore* user = new JsonPrefStore(
+      pref_filename,
+      BrowserThread::GetMessageLoopProxyForThread(BrowserThread::FILE));
   DefaultPrefStore* default_pref_store = new DefaultPrefStore();
 
   PrefNotifierImpl* pref_notifier = new PrefNotifierImpl();