Fix a race condition in preference metric reporting.

base/prefs/pref_service_builder.cc

Index: base/prefs/pref_service_builder.cc
diff --git a/base/prefs/pref_service_builder.cc b/base/prefs/pref_service_builder.cc
index 16b4565663b4ce67a7239d03d191b1d0b68e66bb..d4f1e89329c05b2d2b879a9a69c366bdebce1aaf 100644
--- a/base/prefs/pref_service_builder.cc
+++ b/base/prefs/pref_service_builder.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"
 
@@ -69,7 +70,9 @@ PrefServiceBuilder& PrefServiceBuilder::WithReadErrorCallback(
 PrefServiceBuilder& PrefServiceBuilder::WithUserFilePrefs(
     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>());
   return *this;
 }