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

chrome/browser/renderer_host/render_widget_host_view_mac.h

Index: chrome/browser/renderer_host/render_widget_host_view_mac.h
diff --git a/chrome/browser/renderer_host/render_widget_host_view_mac.h b/chrome/browser/renderer_host/render_widget_host_view_mac.h
index cdfee6ab2f0a9dc3772e5355f6b91a6cb4ccab7b..34d57332debc6d262782b62a6fbe994a957b10c4 100644
--- a/chrome/browser/renderer_host/render_widget_host_view_mac.h
+++ b/chrome/browser/renderer_host/render_widget_host_view_mac.h
@@ -104,7 +104,8 @@ class RenderWidgetHostViewMac : public RenderWidgetHostView {
   // 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