It turns out that setting the parent link on a clip chain is no longer
needed (and probably hasn't been since WR started applying a stacking
context's clip to the SC's contents). In fact it can produce incorrect
behaviour in some cases, because it doesn't match the semantics of
Gecko's clip chains. This removes the parent link on the Gecko side and
adds a test for this scenario.
Differential Revision: https://phabricator.services.mozilla.com/D18101
--HG--
extra : moz-landing-system : lando
