Bug 930048 - Remove need to read objects directly when optimizing singleton accesses, r=jandem.
bug 931399 - shutdown DocAccessible's when the related docshell is destroyed r=bz

When we get to PresShell::Destroy the pres shell and document have already
nulled out their poitners to the docshell which DocAccessible::Shutdown()
needs to remove the document from the command observer.