Provide a default implementation for QuicCryptoClientStream::ProofHandler::OnCertificateRequested.

This makes OnCertificateRequested no longer a pure virtual function, allowing subclasses to opt out of implementing it. The default implementation returns false, indicating that the handshake should not be suspended.

This allows temporarily unblocking the roll to Chromium.

Protected by quic_restart_flag_quic_client_cert_support.

PiperOrigin-RevId: 917940015

quiche/quic/core/quic_crypto_client_stream.h

diff --git a/quiche/quic/core/quic_crypto_client_stream.h b/quiche/quic/core/quic_crypto_client_stream.h
index b30c199..0d419d3 100644
--- a/quiche/quic/core/quic_crypto_client_stream.h
+++ b/quiche/quic/core/quic_crypto_client_stream.h
@@ -272,7 +272,9 @@
     // contains the DER-encoded requested CAs. Returns true if the handshake
     // should be suspended to provide the certificate asynchronously.
     virtual bool OnCertificateRequested(
-        const std::vector<std::string>& cert_authorities) = 0;
+        const std::vector<std::string>& /*cert_authorities*/) {
+      return false;
+    }
   };
 
   QuicCryptoClientStream(const QuicServerId& server_id, QuicSession* session,