Merge <use> instance association and removal of disallowed elements

If we remove an element because it's disallowed during <use> shadow tree
construction, we won't clear the corresponding element reference that we
set up just before. This can cause the DCHECK in CorrespondingElement() to trigger, because the element will have a corresponding element, but not be part of a shadow root.

Merge the AssociateCorrespondingElements() and
RemoveDisallowedElementsFromSubtree() actions into a single subtree walk
that performs both of the actions at the same time. This is marginally
more complex and prevents a disallowed element from being associated
with its corresponding element.

Fixed: 1371700
Change-Id: I21c963d898716b1ddf1486baf07b383c74a24dc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3936288
Commit-Queue: Philip Rogers <pdr@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/main@{#1055940}