function saveBackgroundColor(url, color) {
  var items = {};
  items[url] = color;
  // See https://developer.chrome.com/apps/storage#type-StorageArea. We omit the
  // optional callback since we don't need to perform any action once the
  // background color is saved.
  chrome.storage.sync.set(items);
}
Sets the given background color for url.

@param {string} url URL for which background color is to be saved.
@param {string} color The background color to be saved.