reset VisibleSelection at the Undo command.

Source/core/editing/VisibleSelection.h

 /*
  * Copyright (C) 2004 Apple Computer, 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 114 matching lines @@
         virtual void didChangeVisibleSelection() = 0;
         virtual void trace(Visitor*) { }
     };
 
     void setChangeObserver(ChangeObserver&);
     void clearChangeObserver();
     void didChange(); // Fire the change observer, if any.
 
     void trace(Visitor*);
 
+    void validatePositionsIfNeeded();
+
 #ifndef NDEBUG
     void debugPosition() const;
     void formatForDebugger(char* buffer, unsigned length) const;
     void showTreeForThis() const;
 #endif
 
 private:
     void validate(TextGranularity = CharacterGranularity);
 
     // Support methods for validate()
@@ skipping 38 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const WebCore::VisibleSelection&);
 void showTree(const WebCore::VisibleSelection*);
 #endif
 
 #endif // VisibleSelection_h