Integrate registry_hash_store_contents with the rest of tracked prefs.

components/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 COMPONENTS_PREFS_PREF_FILTER_H_
 #define COMPONENTS_PREFS_PREF_FILTER_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 30 matching lines @@
       const PostFilterOnLoadCallback& post_filter_on_load_callback,
       std::unique_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. Modifications to
   // |pref_store_contents| will be persisted to disk and also affect the
-  // in-memory state.
-  virtual void FilterSerializeData(
+  // in-memory state. If the returned Callback is non-null, it will be
+  // registered to be invoked synchronously after the next write (from the I/O
+  // TaskRunner so it must not be bound to thread-unsafe member state).
+  virtual base::Callback<void(bool)> FilterSerializeData(

[gab, 2016/09/23 14:01:18]

As mentioned in post-commit of other CL, I think this can be done in a precursor
CL, probably mixable with the other required change we discussed.

[proberge, 2016/09/23 15:24:34]

I'll add that to https://codereview.chromium.org/2366743002/

       base::DictionaryValue* pref_store_contents) = 0;
 };
 
 #endif  // COMPONENTS_PREFS_PREF_FILTER_H_