Use worker pool for IO in JsonPrefStore.

chrome/service/service_process_prefs.cc

Index: chrome/service/service_process_prefs.cc
diff --git a/chrome/service/service_process_prefs.cc b/chrome/service/service_process_prefs.cc
index 511709a7d8f5d5893f0f78d4a6c2b516bf62f24f..e259e2c91db77c9e2bae2f3b239704125c56e807 100644
--- a/chrome/service/service_process_prefs.cc
+++ b/chrome/service/service_process_prefs.cc
@@ -8,8 +8,8 @@
 
 ServiceProcessPrefs::ServiceProcessPrefs(
     const FilePath& pref_filename,
-    base::MessageLoopProxy* file_message_loop_proxy)
-    : prefs_(new JsonPrefStore(pref_filename, file_message_loop_proxy)) {
+    base::SequencedTaskRunner* blocking_task_runner)
+    : prefs_(new JsonPrefStore(pref_filename, blocking_task_runner)) {
 }
 
 ServiceProcessPrefs::~ServiceProcessPrefs() {}