Add Curve25519 version of pairing authenticators

remoting/protocol/negotiating_host_authenticator.h

Index: remoting/protocol/negotiating_host_authenticator.h
diff --git a/remoting/protocol/negotiating_host_authenticator.h b/remoting/protocol/negotiating_host_authenticator.h
index 25a761ed1db69d7a70fccce2f7daf6802fb7220f..10d378013f97020ebc4f6396685c77e9be3ae90f 100644
--- a/remoting/protocol/negotiating_host_authenticator.h
+++ b/remoting/protocol/negotiating_host_authenticator.h
@@ -31,7 +31,7 @@ class NegotiatingHostAuthenticator : public NegotiatingAuthenticatorBase {
   ~NegotiatingHostAuthenticator() override;
 
   // Creates a host authenticator for It2Me host.
-  static scoped_ptr<Authenticator> CreateForIt2Me(
+  static scoped_ptr<NegotiatingHostAuthenticator> CreateForIt2Me(
       const std::string& local_id,
       const std::string& remote_id,
       const std::string& local_cert,
@@ -41,7 +41,7 @@ class NegotiatingHostAuthenticator : public NegotiatingAuthenticatorBase {
   // Creates a host authenticator, using a fixed PIN. If |pairing_registry| is
   // non-nullptr then the paired methods will be offered, supporting
   // PIN-less authentication.
-  static scoped_ptr<Authenticator> CreateWithPin(
+  static scoped_ptr<NegotiatingHostAuthenticator> CreateWithPin(
       const std::string& local_id,
       const std::string& remote_id,
       const std::string& local_cert,
@@ -50,7 +50,7 @@ class NegotiatingHostAuthenticator : public NegotiatingAuthenticatorBase {
       scoped_refptr<PairingRegistry> pairing_registry);
 
   // Creates a host authenticator, using third party authentication.
-  static scoped_ptr<Authenticator> CreateWithThirdPartyAuth(
+  static scoped_ptr<NegotiatingHostAuthenticator> CreateWithThirdPartyAuth(
       const std::string& local_id,
       const std::string& remote_id,
       const std::string& local_cert,