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

net/quic/crypto/proof_source_chromium_openssl.cc

Index: net/quic/crypto/proof_source_chromium_openssl.cc
diff --git a/net/quic/crypto/proof_source_chromium_openssl.cc b/net/quic/crypto/proof_source_chromium_openssl.cc
index c89f2d9d89bb5eda72a1f4f40a51850aec967ff4..0563c8c2b7e0453ab5f90d5a5a97a4d952508dac 100644
--- a/net/quic/crypto/proof_source_chromium_openssl.cc
+++ b/net/quic/crypto/proof_source_chromium_openssl.cc
@@ -72,7 +72,8 @@ 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) {
   DCHECK(private_key_.get()) << " this: " << this;
 
   crypto::OpenSSLErrStackTracer err_tracer(FROM_HERE);