Bug 1637300 - Further reduce unnecessary scene building. r=gw
Bug 1637300 - Further reduce unnecessary scene building. r=gw

Differential Revision: https://phabricator.services.mozilla.com/D74881

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/39a5c6cb5884941bd21f84d21597eb5b42ddf43d

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