Remove support for custom pseudo elements.

Source/core/dom/Element.cpp

Index: Source/core/dom/Element.cpp
diff --git a/Source/core/dom/Element.cpp b/Source/core/dom/Element.cpp
index 6603f42fd0700ea5c6fe48be28e9464b15a89ea1..032e275db8b73be32eb51cc8ce8f2f3095481fe3 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -1014,8 +1014,6 @@ void Element::attributeChanged(const QualifiedName& name, const AtomicString& ne
         classAttributeChanged(newValue);
     } else if (name == HTMLNames::nameAttr) {
         setHasName(!newValue.isNull());
-    } else if (name == HTMLNames::pseudoAttr) {
-        shouldInvalidateStyle |= testShouldInvalidateStyle && isInShadowTree();
     }
 
     invalidateNodeListCachesInAncestors(&name, this);
@@ -2440,11 +2438,16 @@ String Element::textFromChildren()
 
 const AtomicString& Element::shadowPseudoId() const
 {
-    return getAttribute(pseudoAttr);
+    if (ShadowRoot* root = containingShadowRoot()) {
+        if (root->type() == ShadowRoot::UserAgentShadowRoot)
+            return fastGetAttribute(pseudoAttr);
+    }
+    return nullAtom;
 }
 
 void Element::setShadowPseudoId(const AtomicString& id)
 {
+    ASSERT(CSSSelector::parsePseudoType(id) == CSSSelector::PseudoWebKitCustomElement || CSSSelector::parsePseudoType(id) == CSSSelector::PseudoUserAgentCustomElement);
     setAttribute(pseudoAttr, id);
 }