Fix problems with render_widget_host_ being deleted out-of-sync with cocoa_view_.

chrome/browser/renderer_host/render_widget_host_view_mac.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_MAC_H_
 #define CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_MAC_H_
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/scoped_nsobject.h"
@@ skipping 86 matching lines @@
                                   int selected_item,
                                   const std::vector<WebMenuItem>& items);
 
   void KillSelf();
 
   void set_parent_view(BaseView* parent_view) { parent_view_ = parent_view; }
 
   // These member variables should be private, but the associated ObjC class
   // needs access to them and can't be made a friend.
 
-  // The associated Model.
+  // The associated Model.  Can be NULL if Destroy() is called when
+  // someone (other than superview) has retained |cocoa_view_|.
   RenderWidgetHost* render_widget_host_;
 
   // This is true when we are currently painting and thus should handle extra
   // paint requests by expanding the invalid rect rather than actually painting.
   bool about_to_validate_and_paint_;
 
   // This is the rectangle which we'll paint.
   NSRect invalid_rect_;
 
   // The time at which this view started displaying white pixels as a result of
@@ skipping 33 matching lines @@
   // Factory used to safely scope delayed calls to ShutdownHost().
   ScopedRunnableMethodFactory<RenderWidgetHostViewMac> shutdown_factory_;
 
   // Used for positioning a popup menu.
   BaseView* parent_view_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostViewMac);
 };
 
 #endif  // CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_MAC_H_