Introduce a hash_of_hashes dictionary.

chrome/browser/prefs/pref_hash_store.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_STORE_H_
 #define CHROME_BROWSER_PREFS_PREF_HASH_STORE_H_
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
 
 namespace base {
 class Value;
 }  // namespace base
 
+namespace internals {
+
+// Hash of hashes for each profile, used to validate the existing hashes when
+// debating whether an unknown value is to be trusted, will be stored as a
+// string under
+// |kProfilePreferenceHashes|.|kHashOfHashesPref|.|hash_stored_id_|.
+const char kHashOfHashesPref[] = "hash_of_hashes";
+
+}  // namespace internals
+
 // 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 ValueState {
     // 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.
-    UNKNOWN_VALUE,
+    UNTRUSTED_UNKNOWN_VALUE,
+    // No stored hash exists for the preference value, but the current set of
+    // hashes stored is trusted and thus this value can safely be seeded. This
+    // happens when all hashes are already properly seeded and a newly
+    // tracked value needs to be seeded). NULL values are inherently trusted as
+    // well.
+    TRUSTED_UNKNOWN_VALUE,
   };
 
   // Checks |initial_value| against the existing stored value hash.
   virtual ValueState CheckValue(
       const std::string& path, const base::Value* initial_value) const = 0;
 
   // Stores a hash of the current |value| of the preference at |path|.
   virtual void StoreHash(const std::string& path,
                          const base::Value* value) = 0;
 };
 
 #endif  // CHROME_BROWSER_PREFS_PREF_HASH_STORE_H_