Implement :defined pseudo-class selector for Custom Elements

third_party/WebKit/Source/core/css/SelectorChecker.cpp

Index: third_party/WebKit/Source/core/css/SelectorChecker.cpp
diff --git a/third_party/WebKit/Source/core/css/SelectorChecker.cpp b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
index c9c325e9e9c8571d74efbd526992d0a6b11207d0..5cb5611f23db6b101b376afa14bc0e62909b76ac 100644
--- a/third_party/WebKit/Source/core/css/SelectorChecker.cpp
+++ b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
@@ -916,6 +916,9 @@ bool SelectorChecker::checkPseudoClass(const SelectorCheckingContext& context, M
         return context.scope == &element;
     case CSSSelector::PseudoUnresolved:
         return element.isUnresolvedV0CustomElement();
+    case CSSSelector::PseudoDefined:
+        DCHECK(RuntimeEnabledFeatures::customElementsV1Enabled());
+        return element.isDefined();
     case CSSSelector::PseudoHost:
     case CSSSelector::PseudoHostContext:
         return checkPseudoHost(context, result);