Add a new GetNegotiatedProtocol method to StreamSocket and implement in all subclasses.

net/socket/tcp_client_socket_libevent.cc

Index: net/socket/tcp_client_socket_libevent.cc
diff --git a/net/socket/tcp_client_socket_libevent.cc b/net/socket/tcp_client_socket_libevent.cc
index 9b47249a355cd41710e5c16fb9a64b0953b95814..a88654415cf9fc7ab137e81f4034217d73b8b5eb 100644
--- a/net/socket/tcp_client_socket_libevent.cc
+++ b/net/socket/tcp_client_socket_libevent.cc
@@ -733,4 +733,8 @@ base::TimeDelta TCPClientSocketLibevent::GetConnectTimeMicros() const {
   return connect_time_micros_;
 }
 
+NextProto TCPClientSocketLibevent::GetNegotiatedProtocol() const {
+  return kProtoUnknown;
+}
+
 }  // namespace net