DNS: Add stubs for persisting data across restarts

net/dns/host_resolver.h

Index: net/dns/host_resolver.h
diff --git a/net/dns/host_resolver.h b/net/dns/host_resolver.h
index 5c371e43ad533e66ed0f366c36f5f17bb73e1bf7..2082417e19a4ae28d723954b7fd7fb46a4bb8074 100644
--- a/net/dns/host_resolver.h
+++ b/net/dns/host_resolver.h
@@ -186,6 +186,12 @@ class NET_EXPORT HostResolver {
   // nullptr if it's configured to always use the system host resolver.
   virtual std::unique_ptr<base::Value> GetDnsConfigAsValue() const;
 
+  typedef base::Callback<void(std::unique_ptr<const base::Value>)>
+      PersistCallback;
+  virtual void InitializePersistence(
+      const PersistCallback& persist_callback,
+      std::unique_ptr<const base::Value> old_data);
+
   // Creates a HostResolver implementation that queries the underlying system.
   // (Except if a unit-test has changed the global HostResolverProc using
   // ScopedHostResolverProc to intercept requests to the system).