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 c5494b7a68e63a7d40b221e02b9bddf4cc3f895a..8af61f170f15a000b8c028e01ddffdbf55dbe44d 100644
--- a/net/dns/host_resolver.h
+++ b/net/dns/host_resolver.h
@@ -186,6 +186,16 @@ 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;
+  // Configures the HostResolver to be able to persist data (e.g. observed
+  // performance) between sessions. |persist_callback| is a callback that will
+  // be called when the HostResolver wants to persist data; |old_data| is the
+  // data last persisted by the resolver on the previous session.
+  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).