Implement the Custom Elements :unresolved pseudoclass

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 1c68cc87d1ad9a35ddd279caa8940bfe7045fe16..18a589f484746ee4a5e462981f9ad322c4296544 100644
--- a/Source/core/css/SelectorChecker.cpp
+++ b/Source/core/css/SelectorChecker.cpp
@@ -32,6 +32,7 @@
 #include "core/css/CSSSelector.h"
 #include "core/css/CSSSelectorList.h"
 #include "core/css/SiblingTraversalStrategies.h"
+#include "core/dom/CustomElementRegistry.h"
 #include "core/dom/Document.h"
 #include "core/dom/NodeRenderStyle.h"
 #include "core/dom/ShadowRoot.h"
@@ -715,6 +716,11 @@ bool SelectorChecker::checkOne(const SelectorCheckingContext& context, const Sib
                 break;
             }
 
+        case CSSSelector::PseudoUnresolved:
+            if (element->isUnresolvedCustomElement())
+                return true;
+            break;
+
         case CSSSelector::PseudoHorizontal:
         case CSSSelector::PseudoVertical:
         case CSSSelector::PseudoDecrement: