Fix a race condition in preference metric reporting.

base/prefs/pref_filter.h

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_PREFS_PREF_FILTER_H_
+#define BASE_PREFS_PREF_FILTER_H_
+
+#include <string>
+
+#include "base/prefs/base_prefs_export.h"
+
+namespace base {
+class DictionaryValue;
+class Value;
+}  // namespace base
+
+// Filters preferences as they are loaded from disk or updated at runtime.
+// Currently supported only by JsonPrefStore.
+class BASE_PREFS_EXPORT PrefFilter {
+ public:
+  virtual ~PrefFilter() {}
+
+  // Receives notification when the pref store data has been loaded but before
+  // Observers are notified.
+  // Changes made by a PrefFilter during FilterOnLoad do not result in
+  // notifications to |PrefStore::Observer|s.
+  virtual void FilterOnLoad(base::DictionaryValue* pref_store_contents) = 0;
+
+  // Receives notification when a pref store value is changed, before Observers
+  // are notified.
+  virtual void FilterUpdate(const std::string& path,
+                            const base::Value* value) = 0;
+};
+
+#endif  // BASE_PREFS_PREF_FILTER_H_