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

net/quic/crypto/proof_source_chromium_nss.cc

Index: net/quic/crypto/proof_source_chromium_nss.cc
diff --git a/net/quic/crypto/proof_source_chromium_nss.cc b/net/quic/crypto/proof_source_chromium_nss.cc
index f73a6ac0a42c352f9ecc5811084766b0266fc7b0..01fac20d99fd84101a310e1de55d38da657e000b 100644
--- a/net/quic/crypto/proof_source_chromium_nss.cc
+++ b/net/quic/crypto/proof_source_chromium_nss.cc
@@ -23,7 +23,8 @@ bool ProofSourceChromium::GetProof(const IPAddressNumber& server_ip,
                                    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;
 }