Integrate registry_hash_store_contents with the rest of tracked prefs.

components/prefs/json_pref_store.cc

Index: components/prefs/json_pref_store.cc
diff --git a/components/prefs/json_pref_store.cc b/components/prefs/json_pref_store.cc
index 884829a7ac0392dd603417fba068896be475d372..c6368dc0c92ad4f880e4607002b59abfe61e638b 100644
--- a/components/prefs/json_pref_store.cc
+++ b/components/prefs/json_pref_store.cc
@@ -462,8 +462,12 @@ bool JsonPrefStore::SerializeData(std::string* output) {
 
   write_count_histogram_.RecordWriteOccured();
 
-  if (pref_filter_)
-    pref_filter_->FilterSerializeData(prefs_.get());
+  if (pref_filter_) {
+    base::Callback<void(bool)> pref_filter_post_write_callback =
+        pref_filter_->FilterSerializeData(prefs_.get());
+    if (!pref_filter_post_write_callback.is_null())
+      RegisterOnNextWriteSynchronousCallback(pref_filter_post_write_callback);
+  }
 
   JSONStringValueSerializer serializer(output);
   // Not pretty-printing prefs shrinks pref file size by ~30%. To obtain