Sanitize headers in Proxy Authentication Required responses

BUG=431504

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

Cr-Commit-Position: refs/heads/master@{#310014}
const BoundNetLog& HttpProxyClientSocket::NetLog() const {
  return net_log_;
}

const BoundNetLog& HttpProxyClientSocket::NetLog() const {
  return net_log_;
}
