part 2: Make HTMLEditor clean up unnecessary padding line breaks when deleting something immediately before it
Unfortunately, the code does not work well only in white-space: pre-line.
The rending itself is broken, so, it won't work well on Firefox anyway and
this style is not so useful with editable content. Therefore, this patch
does not fix the issue.
Differential Revision: https://phabricator.services.mozilla.com/D225039
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1923251
gecko-commit: 7a9e2f6ed76e8b106f5d762fde373c081e0063d8
gecko-reviewers: m_kato