function getSavedBackgroundColor(url, callback) {
  // See https://developer.chrome.com/apps/storage#type-StorageArea. We check
  // for chrome.runtime.lastError to ensure correctness even when the API call
  // fails.
  chrome.storage.sync.get(url, (items) => {
    callback(chrome.runtime.lastError ? null : items[url]);
  });
}
Gets the saved background color for url.

@param {string} url URL whose background color is to be retrieved.
@param {function(string)} callback called with the saved background color for
    the given url on success, or a falsy value if no color is retrieved.