Prohibit createShadowRoot({mode: 'open'}) on existing shadow root.

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 85e3cac75909a1c28968bc646f0ac8497a31723a..f1636c5f76c05f3f2850626f0ddef7971be0aa33 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -1828,10 +1828,17 @@ PassRefPtrWillBeRawPtr<ShadowRoot> Element::createShadowRoot(const ScriptState*
     UseCounter::count(document(), UseCounter::ElementCreateShadowRootWithParameter);
 
     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;
+
+    if (shadowRootInitDict.hasMode()) {
+        if (shadowRoot()) {
+            exceptionState.throwDOMException(InvalidStateError, "Shadow root cannot be created on a host which already hosts a shadow tree.");
+            return nullptr;
+        }
+        // TODO(kochi): Add support for closed shadow root. crbug.com/459136
+        if (shadowRootInitDict.mode() == "closed") {
+            exceptionState.throwDOMException(NotSupportedError, "Closed shadow root is not implemented yet.");
+            return nullptr;
+        }
     }
 
     RefPtrWillBeRawPtr<ShadowRoot> shadowRoot = createShadowRoot(exceptionState);