[WebUI] A11y Change cr.ui.List aria-activedescendant to follow lead item

aria-activedescendant should follow the item that has "focus", which is
the lead item, not the selected item when selecting multiples with
Ctrl+ArrowKeys. This allows Chromevox to read the lead item when
navigating.

Change the selected item to have the aria-selected attribute to indicate
to Chromevox what item is selected.

Relevant doc about listbox, aria-selected and aria-activedescendant:
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/listbox_role

Bug: 888639
Change-Id: I67455f749c502083950ac34a721f44fb285b100d
Reviewed-on: https://chromium-review.googlesource.com/c/1476938
Commit-Queue: calamity <calamity@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633207}
1 file changed