Integrate registry_hash_store_contents with the rest of tracked prefs.

components/prefs/pref_filter.h

Index: components/prefs/pref_filter.h
diff --git a/components/prefs/pref_filter.h b/components/prefs/pref_filter.h
index 6ca2197618329bc12a2bd26bf4eaa3c0822b8138..0fe264f982d0d0e68febdeabc6a6f283681ec967 100644
--- a/components/prefs/pref_filter.h
+++ b/components/prefs/pref_filter.h
@@ -48,8 +48,10 @@ class COMPONENTS_PREFS_EXPORT PrefFilter {
   // 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(
       base::DictionaryValue* pref_store_contents) = 0;
 };