Add support of serving SCT on the server side in QUIC, gated by QUIC_VERSION_30.

net/quic/crypto/proof_source_chromium_nss.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/crypto/proof_source_chromium.h"
 
 using std::string;
 using std::vector;
 
 namespace net {
 
 ProofSourceChromium::ProofSourceChromium() {}
 
 ProofSourceChromium::~ProofSourceChromium() {}
 
 bool ProofSourceChromium::Initialize(const base::FilePath& cert_path,
                                      const base::FilePath& key_path) {
   return false;
 }
 
 bool ProofSourceChromium::GetProof(const IPAddressNumber& server_ip,
                                    const string& hostname,
                                    const string& server_config,
                                    bool ecdsa_ok,
                                    const vector<string>** out_certs,
-                                   string* out_signature) {
+                                   string* out_signature,
+                                   string* out_leaf_cert_sct) {
   return false;
 }
 
 }  // namespace net