MD Settings: Creating a <settings-dialog> element.
The new element uses the Light DOM to get populated. Example
<my-new-dialog>
  <settings-dialog>
    <div class='title'>Title goes here</div>
    <div class='body'>Dialog body goes here</div>
    <div class='button-container'>buttons go here</div>
    <div class='footer'>footer content goes here</div>
  <settings-dialog>
</my-new-dialog>
Using the new dialog achieves code re-use for
 1) Positioning title, body and button-container and footer.
 2) Providing an 'x' close button at the top right.
BUG=
591585
Committed: 
https://crrev.com/c671afb138abe2e0c7e2bc69df1fb9da9e224c89
Cr-Commit-Position: refs/heads/master@{#379369}