Properly preserve newlines in GetRenderedText for textframes with white-space:pre-line.
This makes innerText better reflect the actual rendered text, and matches the result
seen in Blink for the testcase.
Differential Revision: https://phabricator.services.mozilla.com/D227164
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1927709
gecko-commit: 3f4f1d264c3a761429711fc03dab813087df4331
gecko-reviewers: jwatt