Simplify inDetach() assert and make it stronger.

Source/core/dom/DocumentLifecycle.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 115 matching lines @@
 
     bool stateAllowsTreeMutations() const;
     bool stateAllowsLayoutTreeMutations() const;
     bool stateAllowsDetach() const;
     bool stateAllowsLayoutInvalidation() const;
     bool stateAllowsLayoutTreeNotifications() const;
 
     void advanceTo(State);
     void ensureStateAtMost(State);
 
+    bool inDetach() const { return m_detachCount; }
     void incrementDetachCount() { m_detachCount++; }
     void decrementDetachCount()
     {
         ASSERT(m_detachCount > 0);
         m_detachCount--;
     }
 
 private:
 #if ENABLE(ASSERT)
     bool canAdvanceTo(State) const;
@@ skipping 39 matching lines @@
 inline bool DocumentLifecycle::stateAllowsLayoutInvalidation() const
 {
     return m_state != InPerformLayout
         && m_state != InCompositingUpdate
         && m_state != InPaintInvalidation;
 }
 
 } // namespace blink
 
 #endif