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

Source/core/dom/Element.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc. All rights reserved.
  *           (C) 2007 Eric Seidel (eric@webkit.org)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1810 matching lines @@
     OriginsUsingFeatures::count(scriptState, document(), OriginsUsingFeatures::Feature::ElementCreateShadowRoot);
     return createShadowRoot(exceptionState);
 }
 
 PassRefPtrWillBeRawPtr<ShadowRoot> Element::createShadowRoot(const ScriptState* scriptState, const ShadowRootInit& shadowRootInitDict, ExceptionState& exceptionState)
 {
     ASSERT(RuntimeEnabledFeatures::createShadowRootWithParameterEnabled());
     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);
 
     if (shadowRootInitDict.hasDelegatesFocus())
         shadowRoot->setDelegatesFocus(shadowRootInitDict.delegatesFocus());
 
     return shadowRoot.release();
 }
 
@@ skipping 1584 matching lines @@
 {
 #if ENABLE(OILPAN)
     if (hasRareData())
         visitor->trace(elementRareData());
     visitor->trace(m_elementData);
 #endif
     ContainerNode::trace(visitor);
 }
 
 } // namespace blink