Sanitize headers in Proxy Authentication Required responses

BUG=431504

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

Cr-Commit-Position: refs/heads/master@{#310014}
  std::string observed_proxy_server_uri() const {
    return observed_proxy_server_uri_;
  }

  std::string observed_proxy_server_uri() const {
    return observed_proxy_server_uri_;
  }
