Make WebView::close not crash and start to fix navigation in SkyShell

sky/engine/core/editing/FrameSelection.cpp

 /*
  * Copyright (C) 2004, 2008, 2009, 2010 Apple 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 1123 matching lines @@
 void FrameSelection::prepareForDestruction()
 {
     m_granularity = CharacterGranularity;
 
     m_caretBlinkTimer.stop();
 
     RenderView* view = m_frame->contentRenderer();
     if (view)
         view->clearSelection();
 
+    m_frame = 0;
+
     setSelection(VisibleSelection(), CloseTyping | ClearTypingStyle | DoNotUpdateAppearance);
 }
 
 void FrameSelection::setStart(const VisiblePosition &pos, EUserTriggered trigger)
 {
     if (m_selection.isBaseFirst())
         setBase(pos, trigger);
     else
         setExtent(pos, trigger);
 }
@@ skipping 469 matching lines @@
     sel.showTreeForThis();
 }
 
 void showTree(const blink::FrameSelection* sel)
 {
     if (sel)
         sel->showTreeForThis();
 }
 
 #endif