Use the two Custom Elements node bits to explicitly represent four states.

Source/core/dom/Node.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 226 matching lines @@
     bool isContainerNode() const { return getFlag(IsContainerFlag); }
     bool isTextNode() const { return getFlag(IsTextFlag); }
     bool isHTMLElement() const { return getFlag(IsHTMLFlag); }
     bool isSVGElement() const { return getFlag(IsSVGFlag); }
 
     bool isPseudoElement() const { return pseudoId() != NOPSEUDO; }
     bool isBeforePseudoElement() const { return pseudoId() == BEFORE; }
     bool isAfterPseudoElement() const { return pseudoId() == AFTER; }
     PseudoId pseudoId() const { return (isElementNode() && hasCustomStyleCallbacks()) ? customPseudoId() : NOPSEUDO; }
 
-    bool isCustomElement() const { return getFlag(IsCustomElement); }
-    void setIsCustomElement();
-    bool isUpgradedCustomElement() const { return getFlag(IsUpgradedCustomElement); }
-    void setIsUpgradedCustomElement();
+    enum CustomElementState {
+        NotCustomElement,
+        UpgradeCandidate,
+        Defined,
+        Upgraded
+    };
+    bool isCustomElement() const { return customElementState() != NotCustomElement; }
+    CustomElementState customElementState() const { return CustomElementState((getFlag(CustomElementBit1) << 1) | getFlag(CustomElementBit0)); }
+    void setCustomElementState(CustomElementState newState);
 
     virtual bool isMediaControlElement() const { return false; }
     virtual bool isMediaControls() const { return false; }
     virtual bool isWebVTTElement() const { return false; }
     virtual bool isAttributeNode() const { return false; }
     virtual bool isCharacterDataNode() const { return false; }
     virtual bool isFrameOwnerElement() const { return false; }
     virtual bool isPluginElement() const { return false; }
 
     // StyledElements allow inline style (style="border: 1px"), presentational attributes (ex. color),
@@ skipping 476 matching lines @@
         HasNameOrIsEditingTextFlag = 1 << 17,
 
         InNamedFlowFlag = 1 << 18,
         HasSyntheticAttrChildNodesFlag = 1 << 19,
         HasCustomStyleCallbacksFlag = 1 << 20,
         HasScopedHTMLStyleChildFlag = 1 << 21,
         HasEventTargetDataFlag = 1 << 22,
         V8CollectableDuringMinorGCFlag = 1 << 23,
         IsInsertionPointFlag = 1 << 24,
         IsInShadowTreeFlag = 1 << 25,
-        IsCustomElement = 1 << 26,
+        CustomElementBit0 = 1 << 26,

[dglazkov, 2013/07/21 18:28:27]

I am not excited about Bit0/Bit1 notation. We avoided opacity in these flag
names thus far. Can we possibly avoid it here, too? :)

 
         NotifyRendererWithIdenticalStyles = 1 << 27,
 
-        IsUpgradedCustomElement = 1 << 28,
+        CustomElementBit1 = 1 << 28,

[dglazkov, 2013/07/21 18:28:27]

If they're closely related, maybe they should live together?

 
         DefaultNodeFlags = IsParsingChildrenFinishedFlag
     };
 
     // 3 bits remaining
 
     bool getFlag(NodeFlags mask) const { return m_nodeFlags & mask; }
     void setFlag(bool f, NodeFlags mask) const { m_nodeFlags = (m_nodeFlags & ~mask) | (-(int32_t)f & mask); } 
     void setFlag(NodeFlags mask) const { m_nodeFlags |= mask; } 
     void clearFlag(NodeFlags mask) const { m_nodeFlags &= ~mask; } 
@@ skipping 173 matching lines @@
 
 } //namespace
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const WebCore::Node*);
 void showNodePath(const WebCore::Node*);
 #endif
 
 #endif