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

net/quic/crypto/proof_source_chromium.h

Index: net/quic/crypto/proof_source_chromium.h
diff --git a/net/quic/crypto/proof_source_chromium.h b/net/quic/crypto/proof_source_chromium.h
index ef5e6768c3419891d59126ac6808716e11349654..5fab1385c38cd33aa432c8891a6b0d7a664c7732 100644
--- a/net/quic/crypto/proof_source_chromium.h
+++ b/net/quic/crypto/proof_source_chromium.h
@@ -36,7 +36,8 @@ class NET_EXPORT_PRIVATE ProofSourceChromium : public ProofSource {
                 const std::string& server_config,
                 bool ecdsa_ok,
                 const std::vector<std::string>** out_certs,
-                std::string* out_signature) override;
+                std::string* out_signature,
+                std::string* out_leaf_cert_sct) override;
 
  private:
   scoped_ptr<crypto::RSAPrivateKey> private_key_;