Bug 1646567 - fix nsDocShell::HasUnloadedParent to work for OOP frames. r=nika

With fission enabled, walking the docshell tree to find unloaded
parents would stop at the first OOP parent. Instead we now walk
the BrowsingContext/WindowContext tree. To still check parents
which are OOP, we take advantage of the fact that a parent
WindowContext will no longer be the current WindowContext if
the parent was unloaded. For in process docshell's we additionally
check the docshell directly.

Differential Revision: https://phabricator.services.mozilla.com/D85328