Fix screenreader behavior on site settings subpages.

On site settings subpages (chrome://settings/content/location for
example), in the list of allowed and blocked sites, there are several
issues with screenreaders:
1. The important context of whether a given site is allowed or blocked
   is never read.
2. Site URLs are read multiple times.
3. The list must be navigated with the arrow keys, but this is never
   indicated by the screenreader.

To fix this, this patch refactors the HTML structure and adds a "grid"
role to the entire list, similar to how chrome://bookmarks works. This
works because the screenreader will announce that focus has entered a
grid and read the grid label, which gives context and prompts the user
to use the arrow keys.

However, if the grid contains only a single row, the screenreader will
not treat the element as a grid. To fix this, we detect the singleton
row case and manually add the context to the aria-label attribute.

This patch only changes the generic site-list element. There are
several custom versions of this element, and will need to be changed in
follow-up patches. The element's visuals should remain unchanged.

Bug: 40280860
Change-Id: Ib388aabada7c1c33f39eb5dfd3b2525d68d7a2b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6530820
Reviewed-by: Filipa Senra <fsenra@google.com>
Auto-Submit: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Rainhard Findling <rainhard@chromium.org>
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Commit-Queue: Filipa Senra <fsenra@google.com>
Cr-Commit-Position: refs/heads/main@{#1459937}