Integrate registry_hash_store_contents with the rest of tracked prefs.

components/prefs/json_pref_store_unittest.cc

Index: components/prefs/json_pref_store_unittest.cc
diff --git a/components/prefs/json_pref_store_unittest.cc b/components/prefs/json_pref_store_unittest.cc
index 18299e24ddb6995820258a994ea7233f3272ba15..9da62733bc0968ae36d661daf899240dba0aefb9 100644
--- a/components/prefs/json_pref_store_unittest.cc
+++ b/components/prefs/json_pref_store_unittest.cc
@@ -75,8 +75,10 @@ class InterceptingPrefFilter : public PrefFilter {
       const PostFilterOnLoadCallback& post_filter_on_load_callback,
       std::unique_ptr<base::DictionaryValue> pref_store_contents) override;
   void FilterUpdate(const std::string& path) override {}
-  void FilterSerializeData(
-      base::DictionaryValue* pref_store_contents) override {}
+  base::Callback<void(bool)> FilterSerializeData(
+      base::DictionaryValue* pref_store_contents) override {
+    return base::Callback<void(bool)>();
+  }
 
   bool has_intercepted_prefs() const { return intercepted_prefs_ != NULL; }