REGRESSION(273934@main): [ Debug ] Multiple layout tests crashing.

https://bugs.webkit.org/show_bug.cgi?id=268650
rdar://122199019

Reviewed by Matthieu Dubet.

This restores the behavior pre-273934@main when getting the computed style of pseudo-elements like "::placeholder" / "::file-selector-button".

getComputedStyle() support for these pseudo-elements is currently broken (and has always been), since we return the element style instead, but this change
makes it so we stop asserting.

* Source/WebCore/page/LocalDOMWindow.cpp:
(WebCore::LocalDOMWindow::getComputedStyle const):

Canonical link: https://commits.webkit.org/274010@main

Source/WebCore/page/LocalDOMWindow.cpp

@@ -1612,7 +1612,8 @@ Ref<CSSStyleDeclaration> LocalDOMWindow::getComputedStyle(Element& element, cons
     auto pseudoId = CSSSelector::parsePseudoElement(pseudoElt, CSSSelectorParserContext { element.document() });
     if (!pseudoId)
         return CSSComputedStyleDeclaration::createEmpty(element);
-    return CSSComputedStyleDeclaration::create(element, Style::PseudoElementIdentifier { *pseudoId });
+    // FIXME: CSSSelector::parsePseudoElement should never return PseudoId::None.
+    return CSSComputedStyleDeclaration::create(element, pseudoId == PseudoId::None ? std::nullopt : std::optional(Style::PseudoElementIdentifier { *pseudoId }));
 }
 
 RefPtr<CSSRuleList> LocalDOMWindow::getMatchedCSSRules(Element* element, const String& pseudoElement, bool authorOnly) const