Revert "Reverting custom pseudo element".

Source/core/css/SelectorChecker.cpp

Index: Source/core/css/SelectorChecker.cpp
diff --git a/Source/core/css/SelectorChecker.cpp b/Source/core/css/SelectorChecker.cpp
index 3fc552e4cba3aaa4d8a168bb8228fd9fc76918f3..64c6fd2bcf6d5729ce0352a6464c895ad4eb5d6a 100644
--- a/Source/core/css/SelectorChecker.cpp
+++ b/Source/core/css/SelectorChecker.cpp
@@ -69,11 +69,12 @@ SelectorChecker::SelectorChecker(Document& document, Mode mode)
 static bool matchesCustomPseudoElement(const Element* element, const CSSSelector& selector)
 {
     ShadowRoot* root = element->containingShadowRoot();
-    if (!root || root->type() != ShadowRoot::UserAgentShadowRoot)
+    if (!root)
         return false;
-
     if (element->shadowPseudoId() != selector.value())
         return false;
+    if (selector.pseudoType() == CSSSelector::PseudoWebKitCustomElement && root->type() != ShadowRoot::UserAgentShadowRoot)
+        return false;
 
     return true;
 }