jax` element to
submit the form data to the action URL.

  Example:

    <form is="iron-form" id="form" method="post" action="/form/handler">
      <paper-input name="name" label="name"></paper-input>
      <input name="address">
      ...
    </form>

By default, a native `<button>` element will submit this form. However, if you
want to submit it from a custom element's click handler, you need to explicitly
call the form's `submit` method.

  Example:

    <paper-button raised onclick="submitForm()">Submit