This alters nsIWebBrowserPersistable so that startPersistence takes an
outerWindowID. This allows us to target a particular subframe from
beneath an nsFrameLoader, which is useful when attempting to Save
Frame As a remote browser.
This restores the behavior from before bug 1101100, when various
indirect properties of the document were obtained by JS that ignored
exceptions by using null instead. This is currently breaking for the
documents created by Print Preview, because they have no page descriptor.
This patch also makes similar changes to the contentDescriptor getter.
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.
