Revert "Reverting custom pseudo element".

Source/core/css/SelectorChecker.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 51 matching lines @@
 SelectorChecker::SelectorChecker(Document& document, Mode mode)
     : m_strictParsing(!document.inQuirksMode())
     , m_documentIsHTML(document.isHTMLDocument())
     , m_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;
 }
 
 Element* SelectorChecker::parentElement(const SelectorCheckingContext& context, bool allowToCrossBoundary) const
 {
     // CrossesBoundary means we don't care any context.scope. So we can walk up from a shadow root to its shadow host.
     if (allowToCrossBoundary)
         return context.element->parentOrShadowHostElement();
 
@@ skipping 995 matching lines @@
     return element.focused() && isFrameFocused(element);
 }
 
 template
 SelectorChecker::Match SelectorChecker::match(const SelectorCheckingContext&, const DOMSiblingTraversalStrategy&, MatchResult*) const;
 
 template
 SelectorChecker::Match SelectorChecker::match(const SelectorCheckingContext&, const ShadowDOMSiblingTraversalStrategy&, MatchResult*) const;
 
 }