it is scrollable and it is not
the last child in its parent container, indicating there is more content below. The bottom divider
is hidden if it is scrolled to the bottom.

If `paper-dialog-scrollable` is not a direct child of the element implementing `Polymer.PaperDialogBehavior`,
remember to set the `dialogElement`:

    <paper-dialog-impl id="myDialog">
      <h2>Header</h2>
      <div class="my-content-wrapper">
        <h4>Sub-header</h4>
        <paper-dialog-scrollable>
          Lorem ipsum...
    