Add ShadowRootInit dictionary.

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 33e363250550cde42777781a8a60cb984a27d45b..77cdcf9e400f6b33ba56454926f09664b12a67bc 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -67,6 +67,7 @@
 #include "core/dom/PseudoElement.h"
 #include "core/dom/ScriptableDocumentParser.h"
 #include "core/dom/SelectorQuery.h"
+#include "core/dom/ShadowRootInit.h"
 #include "core/dom/StyleChangeReason.h"
 #include "core/dom/StyleEngine.h"
 #include "core/dom/Text.h"
@@ -1803,9 +1804,15 @@ CustomElementDefinition* Element::customElementDefinition() const
     return nullptr;
 }
 
-PassRefPtrWillBeRawPtr<ShadowRoot> Element::createShadowRoot(ScriptState* scriptState, ExceptionState& exceptionState)
+PassRefPtrWillBeRawPtr<ShadowRoot> Element::createShadowRoot(ScriptState* scriptState, ShadowRootInit& shadowRootInitDict, ExceptionState& exceptionState)
 {
     OriginsUsingFeatures::count(scriptState, document(), OriginsUsingFeatures::Feature::ElementCreateShadowRoot);
+    // TODO(kochi): Add support for closed shadow root. crbug.com/459136
+    if (shadowRootInitDict.hasMode() && shadowRootInitDict.mode() == "closed") {
+        exceptionState.throwDOMException(NotSupportedError, "Closed shadow root is not implemented yet.");
+        return nullptr;
+    }
+
     return createShadowRoot(exceptionState);
 }