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[WPT] Various test cases in custom-elements/registries/element-mutation.html fail
https://bugs.webkit.org/show_bug.cgi?id=319388

Reviewed by Chris Dumez.

The failure was caused by a built-in element (e.g. div) parsed inside an declarative
shadow root with null registry (e.g. declared with shadowrootcustomelementregistry
content attribute on template) not getting the usesNullCustomElementRegistry flag -
only custom elements and unknown elements did (in HTMLConstructionSite's
createHTMLElementOrFindCustomElementInterface inside the "if (!element)" block).
The built-in element merely resolved to null via its tree scope.

That worked for the original (parsed before any global registry was ensured), but
broke on cloning: once the global registry exists, cloning the subtree runs
Element::insertionSteps on the cloned nodes which pins the cloned element with
the global registry in the scoped map. Once pinned, registry.initialize(shadowRoot)
could no longer hand it the scoped registry - hence the failures.

This PR applies two fixes to address the failures.

1. HTMLConstructionSite.cpp - set usesNullCustomElementRegistry on all elements
   (built-in included) parsed inside a null-registry tree scope, so an element
   behaves identically to its clones and keeps resolving to null across mutations.
   This unifies the previously separate custom-element and fragment-parsing branches.
2. Element::attachShadow - an imperative attachShadow() with no explicit registry
   now always defaults to the document's registry, instead of null when the host
   carries the null-registry flag. The host stays null; only the new shadow root uses
   the global registry.

(2) is required because (1) gives built-in elements the flag, and without it their
attachShadow default would flip to null. As a bonus, it also fixes the pre-existing
ShadowRoot-init-declarative.html custom-element FAIL, which failed for this exact
reason.

Finally, this PR also addresses a bug that we weren't propagating the parent node's
null-registry-ness to a child node during non-fragment parsing by generalizing
the condition (no longer checks m_isParsingFragment).

Test: imported/w3c/web-platform-tests/custom-elements/registries/element-mutation.html

* LayoutTests/imported/w3c/web-platform-tests/custom-elements/registries/ShadowRoot-init-declarative-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/custom-elements/registries/element-mutation-expected.txt:
* Source/WebCore/dom/Element.cpp:
* Source/WebCore/html/parser/HTMLConstructionSite.cpp:
(WebCore::HTMLConstructionSite::createHTMLElementOrFindCustomElementInterface):

Canonical link: https://commits.webkit.org/317215@main

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