Fix the positioning of the progressive loadingBar

This PR changes the positioning of the progressive loadingBar to address the following issue:
>When the PDF document is loaded with zoom set to 'page-fit', once the progressive loading bar is removed, the pages will be slightly shorter than the viewer height. To see this issue, try opening: http://mozilla.github.io/pdf.js/web/viewer.html#page=2&zoom=page-fit.

This PR replaces the sub-optimal approach suggested in #3224.

@yurydelendik Is this a better approach?