set singleProxy(data) {
    var checkbox = document.getElementById('singleProxyForEverything');
    checkbox.checked = !!data;

    if (data)
      this.httpProxy = data;

    if (checkbox.checked)
      checkbox.parentNode.parentNode.classList.add('single');
    else
      checkbox.parentNode.parentNode.classList.remove('single');
  }
@see http://code.google.com/chrome/extensions/trunk/proxy.html
@param {?ProxyServer} data An object containing the proxy server host,
    port, and scheme. If null, the single proxy checkbox will be unchecked.