Fix a race condition in preference metric reporting.

chrome/browser/prefs/pref_hash_tracker.h

+// Copyright 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 CHROME_BROWSER_PREFS_PREF_HASH_TRACKER_H_
+#define CHROME_BROWSER_PREFS_PREF_HASH_TRACKER_H_
+
+#include <string>
+
+namespace base {
+class Value;
+}  // namespace base
+
+// Updates stored preference value hashes in response to preference value
+// changes.
+class PrefHashTracker {
+ public:
+  virtual ~PrefHashTracker() {}
+  
+  // Receives notification of preference value changes.
+  virtual void OnPrefValueChanged(const std::string& path,
+                                  const base::Value* new_value) = 0;
+};
+
+#endif  // CHROME_BROWSER_PREFS_PREF_HASH_TRACKER_H_