Dashboard: Remove use of z-index for ring overlay; use DOM ordering

Dashboard: Remove use of z-index for ring overlay; use DOM ordering
https://bugs.webkit.org/show_bug.cgi?id=148921

Reviewed by Alexey Proskuryakov.

The ring image is positioned above the platform icon in markup and we specify the CSS
property z-index for the ring image so that it is painted on top of the platform icon
instead of below it (by DOM ordering). Instead we can take advantage of the transparency
of the ring image, switch the order of these DOM elements and remove the use of the CSS
property z-index to achieve a similar effect.

* BuildSlaveSupport/build.webkit.org-config/public_html/dashboard/Scripts/Main.js:
(documentReady): Create the platform icon image element before the ring image such that
the ring image is painted on top of the platform icon.
* BuildSlaveSupport/build.webkit.org-config/public_html/dashboard/Styles/Main.css:
(table.queue-grid td.logo img.ring): Removed property z-index.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@189489 268f45cc-cd09-0410-ab3c-d52691b4dbfc