We will use the new type for the generated IPDL message handler
prototype to make sure correct error handling method is called.
MozReview-Commit-ID: AzVbApxFGZ0
The way IPDL glue currently works, it's unsafe to do Send__delete__ on
another actor in that context, and these callbacks can indirectly cause
that; see bug for details.
The high-level overview is that the parts of nsWebBrowserPersist which
access the DOM have been factored out (as WebBrowserPersistLocalDocument)
and abstracted (nsIWebBrowserPersistDocument) such that they can be
implemented in the cross-process case using IPC.
