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

net/curvecp/curvecp_client_socket.cc

Index: net/curvecp/curvecp_client_socket.cc
diff --git a/net/curvecp/curvecp_client_socket.cc b/net/curvecp/curvecp_client_socket.cc
index 52711744daeb8949aa2c9836a1d6d1b885cc0ff3..2992915179c8b3c96843880a83cc594b1becd0ff 100644
--- a/net/curvecp/curvecp_client_socket.cc
+++ b/net/curvecp/curvecp_client_socket.cc
@@ -96,6 +96,10 @@ base::TimeDelta CurveCPClientSocket::GetConnectTimeMicros() const {
   return base::TimeDelta::FromMicroseconds(-1);
 }
 
+NextProto CurveCPClientSocket::GetNegotiatedProtocol() const {
+  return kProtoUnknown;
+}
+
 int CurveCPClientSocket::Read(IOBuffer* buf,
                               int buf_len,
                               const CompletionCallback& callback) {