[Interop] Keep visited set from aria-labeledby traversal

In [1] the change was made to use a copy of the visited set for
the aria-labeledby traversal, so that nodes added in that traversal
would not persist in the visited set used later in name
computation. This was done for the sake of the name sources reported
to the inspector protocol: the intent seems to have been that if a
node was encountered in the labeledby traversal, it should still
be able to show up as a superseded name source later in the traversal.

This is causing functionality issues however, since it can cause the
text of a node to be used twice, for example in the test
comp_name_from_content.html [2] which is part of Interop2024.

Hence in this CL, stop using a thrown-away copy of the visited set
for the aria-labeledby traversal.

Note, this affects some tests in
comp_name_from_content.tentative.html, where there is not yet
consensus on the correct behavior. See discussion in [3]. Depending
on how [3] is settled, we may eventually need to maintain two
"visited" sets:
- One to track the elements we've visited so far in a given labeledby
  traversal, which gets thrown away after that traversal.
- One to track the elements whose text we've used (including in a
  labeledby traversal), which is maintained for the entire name
  calculation.

But until there's consensus on that behavior, I want to avoid this
further complexity. So, this CL aims to do the simplest thing that
aligns us with the subset of behavior agreed on for Interop2024.

[1] https://codereview.chromium.org/1939303002
[2] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/external/wpt/accname/name/comp_name_from_content.html;l=221;drc=bd2cb163ac4cbeb1676c8399c37b92cb7a3db6be
[3] https://github.com/w3c/accname/issues/209


Bug: 338398669
Change-Id: I4fd5e86169fe417a5d99fd1cf37f3ad358aa9c52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5467872
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1308792}