Fix a race condition in preference metric reporting.

chrome/browser/prefs/pref_hash_store.h

Index: chrome/browser/prefs/pref_hash_store.h
diff --git a/chrome/browser/prefs/pref_hash_store.h b/chrome/browser/prefs/pref_hash_store.h
new file mode 100644
index 0000000000000000000000000000000000000000..449056ae6407005668c975681fe4067a84e366e0
--- /dev/null
+++ b/chrome/browser/prefs/pref_hash_store.h
@@ -0,0 +1,51 @@
+// 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_STORE_H_
+#define CHROME_BROWSER_PREFS_PREF_HASH_STORE_H_
+
+#include <string>
+
+#include "base/memory/scoped_ptr.h"
+
+class PrefHashTracker;
+
+namespace base {
+class Value;
+}  // namespace base
+
+// Stores hashes of and verifies preference values. To use, first call
+// |InitializeTrackedValue| with each preference that should be tracked. Then
+// call |OnPrefValueChanged| to update the hash store when preference values
+// change.
+class PrefHashStore {
+ public:
+  virtual ~PrefHashStore() {}
+
+  enum InitializationResult {
+    // The preference value corresponds to its stored hash.
+    UNCHANGED,
+    // The preference has been cleared since the last hash.
+    CLEARED,
+    // The preference value corresponds to its stored hash, which was calculated
+    // using a legacy hash algorithm.
+    MIGRATED,
+    // The preference value has been changed since the last hash.
+    CHANGED,
+    // No stored hash exists for the preference value.
+    INITIALIZED
+  };
+
+  // Configures tracking of the preference named by |path|. Checks
+  // |initial_value| against the existing stored hashes and returns the result.
+  // If the result is not UNCHANGED a new hash will be calculated and stored.
+  virtual InitializationResult InitializeTrackedValue(
+      const std::string& path, const base::Value* initial_value) = 0;
+
+  // 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_STORE_H_