Bug 1679482 - Refactor work for computing external surface dependencies into helper. r=gw
Bug 1679482 - Refactor work for computing external surface dependencies into helper. r=gw

Differential Revision: https://phabricator.services.mozilla.com/D98271

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/f926a9326dd23279eed7622bb1f49374367ac76c

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