Add a stack of queues of elements with reaction queues.

third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h

Index: third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h b/third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h
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index 0000000000000000000000000000000000000000..f906b78d6b19ccf915c9cb9097c3ce9c6d789990
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+++ b/third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h
@@ -0,0 +1,43 @@
+// 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.
+
+#ifndef CustomElementReactionStack_h
+#define CustomElementReactionStack_h
+
+#include "core/CoreExport.h"
+#include "platform/heap/Handle.h"
+#include "wtf/Noncopyable.h"
+
+namespace blink {
+
+class CustomElementReaction;
+class CustomElementReactionQueue;
+class Element;
+
+// https://html.spec.whatwg.org/multipage/scripting.html#custom-element-reactions
+class CORE_EXPORT CustomElementReactionStack final
+    : public GarbageCollectedFinalized<CustomElementReactionStack> {
+    WTF_MAKE_NONCOPYABLE(CustomElementReactionStack);
+public:
+    CustomElementReactionStack();
+    ~CustomElementReactionStack();
+
+    DECLARE_TRACE();
+
+    void push();
+    void popInvokingReactions();
+    void enqueue(Element*, CustomElementReaction*);
+
+private:
+    using ElementReactionQueueMap =
+        HeapHashMap<Member<Element>, Member<CustomElementReactionQueue>>;
+    ElementReactionQueueMap m_map;
+
+    using ElementQueue = HeapVector<Member<Element>, 1>;
+    HeapVector<Member<ElementQueue>> m_stack;
+};
+
+} // namespace blink
+
+#endif // CustomElementReactionStack_h