[scoped-registry] Implement scoped custom element upgrade

[chromium-wpt-export-bot]

This patch:

1. Changes CustomElement::Registry() to return tree-scoped registries
   instead of the global registry. As this function is used by a lot
   of callers (including upgrade), this allows these callers to use
   the scoped registry associated with the tree scope.

2. Custom element construction stack entries are augmented with the
   definition being used, so that later when calling super(), we
   can still know which custom element definition is being used.
   See [scoped-registries] Interaction with HTML element's overridden constructor steps WICG/webcomponents#969 for details.

Bug: 1304439
Change-Id: Id510ecea0f4c5cf6386f77a39d346918c9592e76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4144367
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1137942}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.

[jonathan-j-lee]

Traceback (most recent call last):
  File "./wpt", line 10, in <module>
    wpt.main()
  File "/home/test/web-platform-tests/tools/wpt/wpt.py", line 227, in main
    rv = script(*args, **kwargs)
  File "/home/test/web-platform-tests/tools/wpt/run.py", line 885, in run
    wptrunner_kwargs = setup_wptrunner(venv, **kwargs)
  File "/home/test/web-platform-tests/tools/wpt/run.py", line 853, in setup_wptrunner
    setup_cls.setup(kwargs)
  File "/home/test/web-platform-tests/tools/wpt/run.py", line 190, in setup
    self.setup_kwargs(kwargs)
  File "/home/test/web-platform-tests/tools/wpt/run.py", line 259, in setup_kwargs
    prefs_root = self.browser.install_prefs(kwargs["binary"],
  File "/home/test/web-platform-tests/tools/wpt/browser.py", line 421, in install_prefs
    unzip(get(url).raw, dest=extract_dir)
  File "/home/test/web-platform-tests/tools/wpt/utils.py", line 106, in get
    resp.raise_for_status()
  File "/home/test/web-platform-tests/_venv3/lib/python3.8/site-packages/requests/models.py", line 1021, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 500 Server Error: Internal Server Error for url: https://hg.mozilla.org/mozilla-central/archive/tip.zip/testing/profiles/

Outage should be over. Retrying checks.