Fix a race condition in preference metric reporting.

base/prefs/json_pref_store.h

Index: base/prefs/json_pref_store.h
diff --git a/base/prefs/json_pref_store.h b/base/prefs/json_pref_store.h
index 21fc8f95ac9c0a1863cc665c1ece4133fb14c1d3..ad13feba3f6e1279c7c2bee4d597df35d1698842 100644
--- a/base/prefs/json_pref_store.h
+++ b/base/prefs/json_pref_store.h
@@ -18,6 +18,8 @@
 #include "base/prefs/base_prefs_export.h"
 #include "base/prefs/persistent_pref_store.h"
 
+class PrefFilter;
+
 namespace base {
 class DictionaryValue;
 class FilePath;
@@ -41,7 +43,8 @@ class BASE_PREFS_EXPORT JsonPrefStore
   // |sequenced_task_runner| is must be a shutdown-blocking task runner, ideally
   // created by GetTaskRunnerForFile() method above.
   JsonPrefStore(const base::FilePath& pref_filename,
-                base::SequencedTaskRunner* sequenced_task_runner);
+                base::SequencedTaskRunner* sequenced_task_runner,
+                scoped_ptr<PrefFilter> pref_filter);
 
   // PrefStore overrides:
   virtual bool GetValue(const std::string& key,
@@ -87,6 +90,7 @@ class BASE_PREFS_EXPORT JsonPrefStore
   // Helper for safely writing pref data.
   base::ImportantFileWriter writer_;
 
+  scoped_ptr<PrefFilter> pref_filter_;
   ObserverList<PrefStore::Observer, true> observers_;
 
   scoped_ptr<ReadErrorDelegate> error_delegate_;