Bug 1752027 - Wrench: miscellaneous bug fixes and code cleanup. r=gw
Bug 1752027 - Wrench: miscellaneous bug fixes and code cleanup. r=gw

Part 4: convert is_root bool newtype for better callsite readability

Differential Revision: https://phabricator.services.mozilla.com/D137370

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/f1766b9903d5608c59e8395643a349d687823ecd

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