Improve line-clamp behavior with padding / nested blocks.
This matches Blink with border / padding in nested blocks, and seems
worth doing even if we paint the nested lines. Follow-up patch will
prevent painting the nested lines.
Differential Revision: https://phabricator.services.mozilla.com/D157572
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1791226
gecko-commit: 1c543fc925a7d0e1dc8b8f57dad98e170c233bed
gecko-reviewers: layout-reviewers, dshin