Upgrade custom elements when inserting a node

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

Index: third_party/WebKit/Source/core/dom/custom/CustomElementUpgradeReaction.cpp
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElementUpgradeReaction.cpp b/third_party/WebKit/Source/core/dom/custom/CustomElementUpgradeReaction.cpp
index 8da5f43b04b0a562c632977f40902cca1a9dd23c..8296c3ab40c53843c270451d94618dba8c536f55 100644
--- a/third_party/WebKit/Source/core/dom/custom/CustomElementUpgradeReaction.cpp
+++ b/third_party/WebKit/Source/core/dom/custom/CustomElementUpgradeReaction.cpp
@@ -16,7 +16,11 @@ CustomElementUpgradeReaction::CustomElementUpgradeReaction(
 
 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