scroll handler omits removing the shadow if there is no header

[pstjvn]

The scroll method exits if there is no 'header distributed nodes' and thus the div#dropShadow element is always visible, regardless of the used mode (for example seamed) which leave the shadow as an artefact if the user simply wants to use the components as container (for it provides easy way to contain larger components and scroll them).

Use pattern looks like this:

<core-header-panel mode="seamed" style="height: 100%">
  <my-larger-component style="height: 3000px"></my-larger-component>
</core-header-panel>

This will render always with a drop shadow at the top.

Not sure if could be considered as a bug as the initial design of the component is to be used with header, but still a very valid use case imho.