Sanitize headers in Proxy Authentication Required responses

BUG=431504

Review URL: https://codereview.chromium.org/769043003

Cr-Commit-Position: refs/heads/master@{#310014}
bool SpdyProxyClientSocket::GetSSLInfo(SSLInfo* ssl_info) {
  bool was_npn_negotiated;
  NextProto protocol_negotiated;
  return spdy_stream_->GetSSLInfo(ssl_info, &was_npn_negotiated,
                                  &protocol_negotiated);
}

bool SpdyProxyClientSocket::GetSSLInfo(SSLInfo* ssl_info) {
  bool was_npn_negotiated;
  NextProto protocol_negotiated;
  return spdy_stream_->GetSSLInfo(ssl_info, &was_npn_negotiated,
                                  &protocol_negotiated);
}
