A reference to a document (and the act of saving it) can outlive the
browser tab it was originally loaded in, and this needs to be reflected
in IPC in order to avoid MsgRouteError crashes; see bug for more info.
Note that we still need to pass the PBrowser when starting persistence,
because that's the only handle the parent has on the top-level document;
see comments in this patch for more info.
Also makes TabChildBase::GetDocument public, because it's just a wrapper
around WebNavigation() which is already public, to avoid code duplication.
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.
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.
