Integrate registry_hash_store_contents with the rest of tracked prefs.

components/prefs/json_pref_store.h

Index: components/prefs/json_pref_store.h
diff --git a/components/prefs/json_pref_store.h b/components/prefs/json_pref_store.h
index 047fe74abcd88857597e7ac0d398274820de5b7d..c376584eb8c2cfd773767285d2d887400fed537b 100644
--- a/components/prefs/json_pref_store.h
+++ b/components/prefs/json_pref_store.h
@@ -106,10 +106,15 @@ class COMPONENTS_PREFS_EXPORT JsonPrefStore
   // cleanup that shouldn't otherwise alert observers.
   void RemoveValueSilently(const std::string& key, uint32_t flags);
 
-  // Registers |on_next_successful_write| to be called once, on the next
+  // Registers |on_next_successful_write_reply| to be called once, on the next
   // successful write event of |writer_|.
-  void RegisterOnNextSuccessfulWriteCallback(
-      const base::Closure& on_next_successful_write);
+  void RegisterOnNextSuccessfulWriteReply(
+      const base::Closure& on_next_successful_write_reply);
+
+  // Registers |on_next_successful_write| to be synchronously invoked on the
+  // next successful write event of |writer|.
+  void RegisterOnNextWriteSynchronousCallback(
+      const base::Callback<void(bool success)>& on_next_write_callback);
 
   void ClearMutableValues() override;