Upgrade custom elements when inserting a node

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/dom/custom/CustomElementUpgradeReaction.h"
 
 #include "core/dom/custom/CustomElementDefinition.h"
 
 namespace blink {
 
 CustomElementUpgradeReaction::CustomElementUpgradeReaction(
     CustomElementDefinition* definition)
     : CustomElementReaction(definition)
 {
 }
 
 void CustomElementUpgradeReaction::invoke(Element* element)
 {
-    m_definition->upgrade(element);
+    // Don't call upgrade() if it's already upgraded. Multiple upgrade reactions
+    // could be enqueued because the state changes in step 10 of upgrades.
+    // https://html.spec.whatwg.org/multipage/scripting.html#upgrades
+    if (element->getCustomElementState() == CustomElementState::Undefined)
+        m_definition->upgrade(element);
 }
 
 } // namespace blink