Sanitize headers in Proxy Authentication Required responses

BUG=431504

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

Cr-Commit-Position: refs/heads/master@{#310014}
  void OnBeforeProxyHeadersSent(const ProxyInfo& proxy_info,
                                HttpRequestHeaders* request_headers) {
    observed_before_proxy_headers_sent_ = true;
    observed_proxy_server_uri_ = proxy_info.proxy_server().ToURI();
  }

  void OnBeforeProxyHeadersSent(const ProxyInfo& proxy_info,
                                HttpRequestHeaders* request_headers) {
    observed_before_proxy_headers_sent_ = true;
    observed_proxy_server_uri_ = proxy_info.proxy_server().ToURI();
  }
