Bug 1779387 - Restore clip root parenting for now r=gfx-reviewers,aosmond
Bug 1779387 - Restore clip root parenting for now r=gfx-reviewers,aosmond

There are some hit-test use cases that still rely on clip-chains
and invalid clip-chain handles inheriting from the root clip-id
for a pipeline.

This will become irrelevant once the clip-tree patches land next
week, but for now we can restore these to fix a regression going
out in a release.

Differential Revision: https://phabricator.services.mozilla.com/D151880

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/3112a6346071785641588fe0c0442990c080d3d4

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender