DNS: Add stubs for persisting data across restarts

net/dns/dns_client.cc

Index: net/dns/dns_client.cc
diff --git a/net/dns/dns_client.cc b/net/dns/dns_client.cc
index 274cefa19d0af2c892ad5f6715cf838e15b1ceec..f22104d45eab4d1c1380332ead11287e37d88083 100644
--- a/net/dns/dns_client.cc
+++ b/net/dns/dns_client.cc
@@ -56,6 +56,18 @@ class DnsClientImpl : public DnsClient {
 
   AddressSorter* GetAddressSorter() override { return address_sorter_.get(); }
 
+  std::unique_ptr<const base::Value> GetPersistentData() const override {
+    if (!session_)
+      return std::unique_ptr<const base::Value>();
+    return session_->GetPersistentData();
+  }
+
+  void ApplyPersistentData(const base::Value& data) override {
+    if (!session_)
+      return;
+    session_->ApplyPersistentData(data);
+  }
+
  private:
   scoped_refptr<DnsSession> session_;
   std::unique_ptr<DnsTransactionFactory> factory_;