Remove ShadowRoot::resetStyleInheritance mechanism

https://bugs.webkit.org/show_bug.cgi?id=260338
rdar://114015755

Reviewed by Alan Baradlay.

UA shadow trees should just use 'all: initial' if needed.

* Source/WebCore/dom/ShadowRoot.cpp:
(WebCore::ShadowRoot::setResetStyleInheritance): Deleted.
* Source/WebCore/dom/ShadowRoot.h:
* Source/WebCore/html/HTMLPlugInElement.cpp:
(WebCore::HTMLPlugInElement::didAddUserAgentShadowRoot):
* Source/WebCore/html/shadow/ShadowPseudoIds.cpp:
(WebCore::ShadowPseudoIds::webkitPluginReplacement): Deleted.

Also remove this non-referenced pseudo element name.

* Source/WebCore/html/shadow/ShadowPseudoIds.h:
* Source/WebCore/html/shadow/YouTubeEmbedShadowElement.cpp:

This is the only client for the mechanism, use 'all:initial' instead.

(WebCore::YouTubeEmbedShadowElement::create):
* Source/WebCore/style/StyleResolver.cpp:
(WebCore::Style::Resolver::State::State):

Canonical link: https://commits.webkit.org/266999@main

Source/WebCore/dom/ShadowRoot.cpp

@@ -218,12 +218,6 @@ bool ShadowRoot::childTypeAllowed(NodeType type) const
     }
 }
 
-void ShadowRoot::setResetStyleInheritance(bool value)
-{
-    // If this was ever changed after initialization, child styles would need to be invalidated here.
-    m_resetStyleInheritance = value;
-}
-
 Ref<Node> ShadowRoot::cloneNodeInternal(Document& targetDocument, CloningOperation type)
 {
     RELEASE_ASSERT(m_mode != ShadowRootMode::UserAgent);

Source/WebCore/dom/ShadowRoot.h

@@ -71,9 +71,6 @@ class ShadowRoot final : public DocumentFragment, public TreeScope {
     WEBCORE_EXPORT Style::Scope& styleScope();
     StyleSheetList& styleSheets();
 
-    bool resetStyleInheritance() const { return m_resetStyleInheritance; }
-    void setResetStyleInheritance(bool);
-
     bool delegatesFocus() const { return m_delegatesFocus; }
     bool containsFocusedElement() const { return m_containsFocusedElement; }
     void setContainsFocusedElement(bool flag) { m_containsFocusedElement = flag; }
@@ -144,7 +141,6 @@ class ShadowRoot final : public DocumentFragment, public TreeScope {
 
     void childrenChanged(const ChildChange&) override;
 
-    bool m_resetStyleInheritance : 1 { false };
     bool m_hasBegunDeletingDetachedChildren : 1 { false };
     bool m_delegatesFocus : 1 { false };
     bool m_isCloneable : 1 { false };

Source/WebCore/html/HTMLPlugInElement.cpp

@@ -261,8 +261,6 @@ void HTMLPlugInElement::didAddUserAgentShadowRoot(ShadowRoot& root)
     if (!m_pluginReplacement || !document().page() || displayState() != PreparingPluginReplacement)
         return;
 
-    root.setResetStyleInheritance(true);
-
     m_pluginReplacement->installReplacement(root);
 
     setDisplayState(DisplayingPluginReplacement);

Source/WebCore/html/shadow/ShadowPseudoIds.cpp

@@ -207,12 +207,6 @@ const AtomString& webkitMeterEvenLessGoodValue()
     return webkitMeterEvenLessGoodValue;
 }
 
-const AtomString& webkitPluginReplacement()
-{
-    static MainThreadNeverDestroyed<const AtomString> webkitPluginReplacement("-webkit-plugin-replacement"_s);
-    return webkitPluginReplacement;
-}
-
 const AtomString& webkitProgressBar()
 {
     static MainThreadNeverDestroyed<const AtomString> webkitProgressBar("-webkit-progress-bar"_s);

Source/WebCore/html/shadow/ShadowPseudoIds.h

@@ -75,8 +75,6 @@ const AtomString& webkitMeterOptimumValue();
 const AtomString& webkitMeterSuboptimumValue();
 const AtomString& webkitMeterEvenLessGoodValue();
 
-const AtomString& webkitPluginReplacement();
-
 const AtomString& webkitProgressBar();
 const AtomString& webkitProgressValue();
 const AtomString& webkitProgressInnerElement();

Source/WebCore/html/shadow/YouTubeEmbedShadowElement.cpp

@@ -37,7 +37,7 @@ WTF_MAKE_ISO_ALLOCATED_IMPL(YouTubeEmbedShadowElement);
 Ref<YouTubeEmbedShadowElement> YouTubeEmbedShadowElement::create(Document& document)
 {
     auto element = adoptRef(*new YouTubeEmbedShadowElement(document));
-    element->setPseudo(ShadowPseudoIds::webkitPluginReplacement());
+    element->setInlineStyleProperty(CSSPropertyAll, CSSValueInitial);
     return element;
 }
 

Source/WebCore/style/StyleResolver.cpp

@@ -99,10 +99,6 @@ class Resolver::State {
         : m_element(&element)
         , m_parentStyle(parentStyle)
     {
-        bool resetStyleInheritance = hasShadowRootParent(element) && downcast<ShadowRoot>(element.parentNode())->resetStyleInheritance();
-        if (resetStyleInheritance)
-            m_parentStyle = nullptr;
-
         auto& document = element.document();
         auto* documentElement = document.documentElement();
         if (!documentElement || documentElement == &element)