Bug 1769953 - Only force tile cache slice for root scroll bar. r=tnikkel
Bug 1769953 - Only force tile cache slice for root scroll bar. r=tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D147370

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/ce5bb779d929027b20d6ba1abc6a6ebbc9fe49d1

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