If I run `./mach wpt testing/web-platform/tests/editing` with just enabling the
new normalizer, I see some assertion failures which are caused by scanning or
updating non-editable regions.
According to the debug result, it's caused by sometimes caret point is suggested
in non-editable node and further handling will use various utilities which
asserts whether the target is editable.
Ideally, we should add assertion into `CaretPoint`, but it's hard and I don't
have enough time to fix every regression which will be found by fuzzing tests
even though most of them are not so important in the real web apps. Therefore,
this fixes only the causes which I found in the WPTs.
Differential Revision: https://phabricator.services.mozilla.com/D238408
de4057edf4