Update identifier map entries and notify if they get removed.

This can happen through DestroyElementMaps()
Based on work by Markus Stange and Edgar Chen.

dom/base/nsDocument.h

@@ -216,6 +216,11 @@ public:
   void RemoveContentChangeCallback(nsIDocument::IDTargetObserver aCallback,
                                 void* aData, bool aForImage);
 
+  /**
+   * Remove all elements and notify change listeners.
+   */
+  void ClearAndNotify();
+
   void Traverse(nsCycleCollectionTraversalCallback* aCallback);
 
   struct ChangeCallback {