See `Polymer.IronFitBehavior`.

  ### Backdrop

  Set the `with-backdrop` attribute to display a backdrop behind the overlay. The backdrop is
  appended to the body and is of type `<iron-overlay-backdrop>`. See its doc page for styling
  options.

  In addition, `with-backdrop` will wrap the focus within the content in the light DOM.
  Override the [`_focusableNodes` getter](#Polymer.IronOverlayBehavior:property-_focusableNodes)
  to achieve a different behavior.

  ### Limitations

  The element is styled 