Add "Failed" custom element state

third_party/WebKit/Source/core/dom/custom/CustomElement.cpp

Index: third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp b/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
index 1156806d5fd6dde1670b8356dbed13ad4effb619..6e2242b3a7aebbb1310e4bc0fc4185ce2e7bd8f1 100644
--- a/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
+++ b/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
@@ -15,6 +15,7 @@
 #include "core/frame/FrameHost.h"
 #include "core/frame/LocalDOMWindow.h"
 #include "core/html/HTMLElement.h"
+#include "core/html/HTMLUnknownElement.h"
 #include "platform/text/Character.h"
 #include "wtf/text/AtomicStringHash.h"
 
@@ -168,6 +169,23 @@ HTMLElement* CustomElement::createUndefinedElement(Document& document, const Qua
     return element;
 }
 
+HTMLElement* CustomElement::createFailedElement(Document& document, const QualifiedName& tagName)
+{
+    DCHECK(shouldCreateCustomElement(document, tagName));
+
+    // "create an element for a token":
+    // https://html.spec.whatwg.org/multipage/syntax.html#create-an-element-for-the-token
+
+    // 7. If this step throws an exception, let element be instead a new element
+    // that implements HTMLUnknownElement, with no attributes, namespace set to
+    // given namespace, namespace prefix set to null, custom element state set
+    // to "failed", and node document set to document.
+
+    HTMLElement* element = HTMLUnknownElement::create(tagName, document);
+    element->setCustomElementState(CustomElementState::Failed);
+    return element;
+}
+
 void CustomElement::enqueue(Element* element, CustomElementReaction* reaction)
 {
     // To enqueue an element on the appropriate element queue