Fix a race condition in preference metric reporting.

base/prefs/pref_service_factory.cc

Index: base/prefs/pref_service_factory.cc
diff --git a/base/prefs/pref_service_factory.cc b/base/prefs/pref_service_factory.cc
index 9c598530c1b45131be188daf895e54b723bb93b3..d644cb1c2b05b4599fe9c20e27b780214fbe1c0d 100644
--- a/base/prefs/pref_service_factory.cc
+++ b/base/prefs/pref_service_factory.cc
@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "base/prefs/default_pref_store.h"
 #include "base/prefs/json_pref_store.h"
+#include "base/prefs/pref_filter.h"
 #include "base/prefs/pref_notifier_impl.h"
 #include "base/prefs/pref_service.h"
 
@@ -37,7 +38,8 @@ PrefServiceFactory::~PrefServiceFactory() {}
 void PrefServiceFactory::SetUserPrefsFile(
     const base::FilePath& prefs_file,
     base::SequencedTaskRunner* task_runner) {
-  user_prefs_ = new JsonPrefStore(prefs_file, task_runner);
+  user_prefs_ = new JsonPrefStore(
+      prefs_file, task_runner, scoped_ptr<PrefFilter>());
 }
 
 scoped_ptr<PrefService> PrefServiceFactory::Create(