handleUpdate_(e) {
    'use strict';
    if (this.cookieKey_) {
      var value = this.textAreaElem_.value;
      if (value) {
        // Strip line breaks.
        value = goog.string.stripNewlines(value);
      }
      if (goog.net.Cookies.getInstance().isValidValue(value)) {
        goog.net.Cookies.getInstance().set(this.cookieKey_, value);
        goog.style.setElementShown(this.valueWarningElem_, false);
      } else {
        goog.style.setElementShown(this.valueWarningElem_, true);
      }
    }
  }
Handles user clicking update button.
@param {!goog.events.Event} e The click event.
@private