handlePopout_(event) {
    if (!utils.isChromeExtensionWindow()) {
      throw new Error('Popouts are only supported in Chrome extension.');
    }
    var boundingRect = this.getElement().getBoundingClientRect();
    var currentWidth = Math.round(boundingRect.width);
    var currentHeight = Math.round(boundingRect.height);
    var currentLeft = window.screenX;
    var currentTop = window.screenY;
    var windowBorder = 15;
    chrome.windows.create(
        {
          url: 'prompt.html?popout#' + this.helperProxy_.getHelperId(),
          focused: false,
          type: 'popup',
          width: currentWidth,
          height: currentHeight + windowBorder,
          left: currentLeft,
          top: currentTop - windowBorder
        },
        goog.bind(function(win) {
          setTimeout(goog.bind(function() {
            var request = /** @type {?messages.BridgeMessageRequest} */ (
                this.panel_.getContent());
            var popouts = this.getPopouts_();
            if (popouts.length > 0) {
              goog.array.forEach(popouts, function(popout) {
                // TODO(koto): Change to chrome.tabs.sendMessage(). Maybe.
                popout.postMessage(request, location.origin);
              });
              chrome.windows.update(win.id, {focused: true});
              this.close();
            } else {
              this.displayFailure_(new Error(
                  'Error occurred when connecting to the new window.'));
            }
          }, this), 500);
        }, this));
  }
Opens the prompt in a new window.
@param {Event} event The event that triggered the popout action.
@private