This is no longer as important, with picture caching. Removing it
will simplify the planned changes to switch to a simpler segment
model based on nine-patch rectangles during scene building.

Differential Revision: https://phabricator.services.mozilla.com/D67792

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/5d5fee77d845a14efd3bb4f3988c5ecb1abbae80