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 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.