reset VisibleSelection at the Undo command.

Source/core/editing/Editor.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  *
  * 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
@@ skipping 700 matching lines @@
 }
 
 void Editor::unappliedEditing(PassRefPtrWillBeRawPtr<EditCommandComposition> cmd)
 {
     EventQueueScope scope;
     m_frame.document()->updateLayout();
 
     dispatchEditableContentChangedEvents(cmd->startingRootEditableElement(), cmd->endingRootEditableElement());
 
     VisibleSelection newSelection(cmd->startingSelection());
+    newSelection.validatePositionsIfNeeded();
     changeSelectionAfterCommand(newSelection, FrameSelection::CloseTyping | FrameSelection::ClearTypingStyle);
 
     m_lastEditCommand = nullptr;
     if (UndoStack* undoStack = this->undoStack())
         undoStack->registerRedoStep(cmd);
     respondToChangedContents(newSelection);
 }
 
 void Editor::reappliedEditing(PassRefPtrWillBeRawPtr<EditCommandComposition> cmd)
 {
@@ skipping 504 matching lines @@
     frame().selection().setShouldShowBlockCursor(m_overwriteModeEnabled);
 }
 
 void Editor::trace(Visitor* visitor)
 {
     visitor->trace(m_lastEditCommand);
     visitor->trace(m_mark);
 }
 
 } // namespace WebCore