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 779c2009730366aba4d8fae689d340b2fb81ad71..9df4f3ac87243055d12d530f5d96313e8d087b31 100644
--- a/components/prefs/json_pref_store.cc
+++ b/components/prefs/json_pref_store.cc
@@ -330,6 +330,14 @@ void JsonPrefStore::RegisterOnNextSuccessfulWriteCallback(
   writer_.RegisterOnNextSuccessfulWriteCallback(on_next_successful_write);
 }
 
+void JsonPrefStore::RegisterOnNextSuccessfulWriteBlockingCallback(
+    const base::Closure& on_next_successful_write) {
+  DCHECK(CalledOnValidThread());
+
+  writer_.RegisterOnNextSuccessfulWriteBlockingCallback(
+      on_next_successful_write);
+}
+
 void JsonPrefStore::ClearMutableValues() {
   NOTIMPLEMENTED();
 }
@@ -401,8 +409,11 @@ bool JsonPrefStore::SerializeData(std::string* output) {
 
   write_count_histogram_.RecordWriteOccured();
 
-  if (pref_filter_)
+  if (pref_filter_) {
+    RegisterOnNextSuccessfulWriteBlockingCallback(
+        pref_filter_->GetPostSerializeCallback(prefs_.get()));
     pref_filter_->FilterSerializeData(prefs_.get());
+  }
 
   JSONStringValueSerializer serializer(output);
   // Not pretty-printing prefs shrinks pref file size by ~30%. To obtain