When the PluginRemoved event is fired when changing locations, it's fired
asynchronously such that the document that the plugin belongs to has already
been unloaded. This was causing the hidden plugin notification to appear in
some cases when users browsed away from documents that had hidden plugins
in them. Now we pass the Principal for the unloading document back to the
parent and do a comparison with the current browser Principal to ensure
that they match before showing the hidden plugin notification.
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