view-transition: DCHECK at-rule not used in UA styles

@view-transitions is currently unimplemented when used from a UA
stylesheet. It would have to be added to
StyleEngine::ApplyUserRuleSetChanges (and possibly elsewhere) to make it
work.

However, since we don't currently need it it's simpler to leave this
as-is. This CL removes some TODOs and adds a DCHECK so that someone
adding the view-transitions at-rule to the UA stylesheet will know it
isn't implemented, rather than having it silently fail.

Bug: 1463966
Change-Id: Iddecd1714ffb70b3e76dc66f3fa2267ec0dd7c17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4943692
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210383}