Move preference MACs to the protected preference stores.

base/prefs/pref_filter.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 BASE_PREFS_PREF_FILTER_H_
 #define BASE_PREFS_PREF_FILTER_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
@@ skipping 27 matching lines @@
   // to external users (see SegregatedPrefStore::ReadPrefs() for an example).
   virtual void FilterOnLoad(
       const PostFilterOnLoadCallback& post_filter_on_load_callback,
       scoped_ptr<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) = 0;
 
   // Receives notification when the pref store is about to serialize data
-  // contained in |pref_store_contents| to a string.
+  // contained in |pref_store_contents| to a string. Modifications to
+  // |pref_store_contents| will be persisted to disk and also affect the
+  // in-memory state.
   virtual void FilterSerializeData(
-      const base::DictionaryValue* pref_store_contents) = 0;
+      base::DictionaryValue* pref_store_contents) = 0;
 };
 
 #endif  // BASE_PREFS_PREF_FILTER_H_